Yotta

- Detect whether the output streams are pipes, and infer --plain if they are (use --colourful to force the use of colour control characters to pipes)
- Exit gracefully if stdout is closed prematurely
- Handle unicode characters in the path to the user's download cache.

New Features
- `yotta start` command (aka `yotta run`) to launch the built program (requires target support)
- Support for additional scripting hooks, including post-install, post-build and pre-publish hooks. See the [docs](http://yottadocs.mbed.com/reference/module.htmlscripts) for more info.

Other Improvements
- Depend on an up-to-date version of [certifi](https://pypi.python.org/pypi/certifi), which includes support for the registry's new SSL certificate.
- Update the required versions of dependencies
- Fix an issue that prevented subdirectories from being used in the `lib` or `bin` properties in module.json
- `yotta debug` (and the new `yotta start`) now choose a sensible default program to launch, instead of failing when none is specified.

- Fix race conditions where one yotta process could remove items from the download cache that another yotta process was using
- Fix shrink-wrap creation with non-trivial testDependencies.
- Fix typo in warning message (thanks bridadan!)

This is a bugfix release to fix `yotta install modulenameversion` support.

New Things
- Now modules (and targets) can specify a dependency on the version of yotta they need (handy for making sure people who try to use your snazzy new module with a neolithic version of yotta see a nice error message... although only for versions of yotta from now on, obviously).
- `yotta shrinkwrap` now support targets, for a more complete lock-down of the things you're building.
- Add VCS description to yotta build info, so your build can know exactly what it was built from (thanks bearsh!)
- Improved warning messages when you `yotta link` something that won't be used.
- Improved messages when there are version errors detected during build.
- Various documentation improvements (thanks bridadan!)

Bugfixes
- Fix various race conditions in the cache, so yotta should now be more stable when several instances are running in parallel.
- Fix crash that could occur in `yotta outdated`

Fixes and improvements including
- It's now possible to `.yotta_ignore` CMake files (useful when adopting existing libraries to yotta)
- Warn when a target has not been explicitly set
- Warn when using `yotta link` to link a module which is not a dependency
- Fix backtrace under python 3 when setting an invalid target
- Fix case where `yotta test` could hang indefinitely (thanks bridadan!)
- Always use the full path to import sub packages (yotta no longer modifies sys.path), which should improve compatibility across different python installations.
- Update and simplify Ubuntu installation instructions
- Various other documentation fixes and improvements