bootstrap: Major refactoring

This commit includes the following:

* Fix syntax errors in Python 3
* Include more docstrings in classes, methods, and functions
* Include unit tests using `unittest`
* Merge implementation of `{rustc,cargo}_out_of_date`
* Merge implementation of `RustBuild.{cargo,rustc}`
* Remove unnecessary source code
* Move all the attributes defined outside of `__init__`
* Remove remaining `%s` from print function
* Remove `WindowsError` reference on non-windows systems
* Rename some variables to be more explicit avoid their meaning
* Run bootstrap tests in the CI process
* Remove non-pythonic getters
* Remove duplicate code in `download_stage0` method
* Reduce the number of branches in `build_bootstrap` method
* Re-raise exception when we cannot execute uname in non-windows systems
* Avoid long lines