Using mini_portile to compile libxml2 and libxslt on any (non-windows) platform.
dependencies.yml is now the central location for library dependencies.

Note that this is for both windows cross-compile and packaged dependencies.
Upgrading libxml2 dependency from 2.7.7 to 2.7.8.

Affects only windows users at this time.
mini_portile is a gem dependency if we're building libxml2.
If we're building libxml2, include the tarballs in the gem manifest.
Upgrade mini_portile to support FTP, and removing my FTP-support hacks.