Yotta

- handle a breaking change in PyGithub 1.26 which caused pulling from branches on github to fail (thanks markus-becker-tridonic-com!)
- fix $program path expansion in test commands to use the absolute path (allows tests to run natively on windwows: thanks noahp!)
- fix setting of yotta settings by environment variables under python 3 (thanks markus-becker-tridonic-com!)
- experimental support for displaying the origin of modules in yotta list (included in `-a` flag, or new `-i` flag).

- resolve links when first loading modules to minimise path lengths (fixes cases where linked modules could make projects unbuildable on windows, including using the mbed OS windows tarball release)
- warn if an executable includes nothing to build

- handle rate-limit-exceeded errors from Github (require the user to authenticate to increase the rate limit)
- don't try to handle WindowsError on non-windows platforms
- forward the mbed user ID to the registry if present in the environment

- discourage publishing of executable applications to the registry, make applications private by default
- correctly inherit scripts from base target descriptions
- catch all errors when trying to get VCS build info, and make them non-fatal
- support ninja being called ninja-build (as it is on Fedora)
- fix cases where logout command would not remove all credentials
- add uncaught exception handler that displays the yotta version at the top of the backtrace

- more verbose and informative `yotta search` command output
- implement `yotta update <dependencyname>`
- allow + in target names
- new yotta cheatsheet available at http://yottadocs.mbed.com/reference/cheatsheet.html (thanks BlackstoneEngineering!)
- ensure that .cmake files are included even if the source directory is empty

Usability improvements, including:
- streamlined yotta init: yotta init will no longer create descriptions that result in warnings, and will ask a shorter set of questions for executables
- download the target description when setting it with the `yotta target` subcommand in an existing module
- fix various cases where user-error resulted in exceptions
- PyPI-specific readme, with the correct syntax

And various other bugfixes and improvements.