Rewrite dependency installation in Python
This commit aims to have the end goal of adding AppVeyor CI support to this
repo, and along the way it ended up meaning that the dependency installation
bits were rewritten in Python. This has a number of benefits:
* Python is more portable than shell
* Python is more readable than shell
* curl is no longer required on Windows (powershell is used for downloads)
There are also a few minor updates made as part of this commit as well:
* The README has been updated in how to build Cargo
* We now use `sudo: false` on Travis for faster builds. This is done by
specifying packages to install instead of installing them ourselves.
* pkg-config is no longer listed as a required program
projects / cargo.git / commit