Mirakuru

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- [style] Corrected code to conform with W503, D210 and E402 linters errors as reported by pylama `6.3.1`.
- [feature] Introduced a hack that kills all subprocesses of executor process.
It requires 'ps xe -ww' command being available in OS otherwise logs error.
- [refactoring] Classes name convention change.
Executor class got renamed into SimpleExecutor and StartCheckExecutor class got renamed into Executor.

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- [feature] Ability to set up custom signal for stopping and killing processes managed by executors.
- [feature] Replaced explicit parameters with keywords for kwargs handled by basic Executor init method.
- [feature] Executor now accepts both list and string as a command.
- [fix] Even it's not recommended to import all but `from mirakuru import *` didn't worked. Now it's fixed.
- [tests] increased tests coverage.
Even test cover 100% of code it doesn't mean they cover 100% of use cases!
- [code quality] Increased Pylint code evaluation.

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- [feature] Introduced PidExecutor that waits for specified file to be created.
- [feature] Provided PyPy compatibility.
- [fix] Closing all resources explicitly.

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- [fix] Kill all children processes of Executor started with shell=True.
- [feature] Executors are now context managers - to start executors for given context.
- [feature] Executor.stopped - context manager for stopping executors for given context.
- [feature] HTTPExecutor and TCPExecutor before .start() check whether port
is already used by other processes and raise AlreadyRunning if detects it.
- [refactoring] Moved python version conditional imports into compat.py module.

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- [fix] Fixed an issue where setting shell to True would execute only part of the command.

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- [fix] Fixed an issue where OutputExecutor would hang, if started process stopped producing output.