Fbuild v0.3 RC 2

Changes since last release

TL;DR: A lot of installation improvements.

• Added support for installation via Polkit. Now, you can just run fbuild install, and the installation part will automatically gain escalated privileges.
• Separated install_destdir and install_prefix to make packaging of Fbuild apps easier.
• #32: Fix a stupid typo (thanks to @rjeschmi).
• Add installation hooks pre_install and post_install.

Single-file version is attached.