Yotta

- Add $build_dir and $target_dir variables for target scripts (thanks marshall!)
- **bugfix**: ensure the script environment is available when running scripts for the top-level module

This is my last release as the maintainer of yotta at ARM (I'm leaving ARM to work on other things).

The future maintenance of yotta at ARM hasn't been resolved yet, but 0xc0170 and jaustin are both co-owners of the PyPI project, and thegecko is looking after the yotta registry and website.

- add more useful error message, and suppress back-trace, for errors caused by expired JSON web tokens (such errors are most likely caused by an inaccurate clock). Alleviates 669

Thanks to ccli8 for reporting.

setuptools 20.6.6 (published this morning) causes a failure when resolving the dependencies of `requests[security]` (e.g. `python -c "import pkg_resources; pkg_resources.require('requests[security]')"`), which yotta does indirectly.

This release includes a workaround so that yotta no longer relies on `pkg_resources` to determine its own version.

New Features
- `yotta link` and `yotta link-target` now accept paths to an existing module/target, making the linking process simpler

Fixes and Other Improvements
- pre and post build script hooks are now respected from the top-level module (Thanks DaMouse404!)
- yotta now handles syntax errors in config files gracefully, instead of exploding
- other minor tweaks and documentation updates

- Fix potential infinite recursion in `yotta link` command
- Fix cases where `yotta target` would print a backtrace for an invalid target name
- Improved support for installing specific versions of targets (`yotta target targetname~1.2.3` is now supported, for example)