Minimalmodbus

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New features:

* Implements reading and writing multiple bits simultaneously.
* Support more byteorders (endianness) for floats and long integers.

Breaking changes:

* Renamed method arguments 'numberOfDecimals', 'numberOfRegisters' to
'number_of_decimals', 'number_of_registers'
* Removed example drivers for Eurotherm 3500 and Omegacn 7500, as I no longer have
access to these instruments for testing. It would great if someone would pick
up support for these instruments in a separate project.
* Requires pyserial 3.0 or later.
* Removed module level constants for default values, as they were confusingly named.

Other fixes:

* Allow slave addresses also in the reserved range (up to 255). Reported by GitHub user gnbl.
* Serial port read and write buffers are cleared before each request to the instrument.
Pull request from GitHub user mrrs6.
* Check whether the serial port is open before trying to open it. Reported by Matthias Bolte.
* Custom exceptions for Modbus errors, by Russ Garrett.
* Silent period between messages is at least 1.75 ms to fulfill Modbus standard. Reported
by GitHub user draput.
* Use time.monotonic if available. Suggested by Matthias Bolte.
* Implemented write timeout, to avoid hanging when writing. Instead it will raise an exception.
Reported by Austin Stover.
* Better checking of number of registers when reading and writing.
* Rename internal methods and variables to be PEP8 compliant.
* Improved documentation.

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* Faster CRC calculation by using a lookup table (thanks to Peter)
* Handling of local echo (thanks to Luca Di Gregorio)
* Improved behavior of dummy_serial (number of bytes to read)
* Improved debug messages (thanks to Dino)
* Using project setup by the cookie-cutter tool.
* Reshuffled source files and documentation.
* Moved source to GitHub from Sourceforge.
* Moved documentation to readthedocs.org
* Using the tox tool for testing on multiple Python versions.
* Using Travis CI test framework
* Using codecov.io code coverage measurement framework
* Added support for Python 3.3 and 3.4.
* Dropped support for Python 2.6.

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* Support for Modbus ASCII mode.

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* Precalculating number of bytes to read, in order to increase the speed.
* Better handling of several instruments on the same serial port, especially
for Windows.
* Improved timing for better compliance with Modbus timing requirements.

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* Read and write multiple registers.
* Read and write floating point values.
* Read and write long integers.
* Read and write strings.
* Support for negative numbers.
* Use of the Python struct module instead of own bit-tweaking internally.
* Improved documentation.

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* Fine-tuned setup.py for smoother installation.
* Improved documentation.