Add caching to yaourt

[alexoneill]

See Issue: #299

The following changes enables caching within yaourt. There are a few things to note:

1. --cachedir fully replaces the default location (currently pacman's cache location, /var/cache/pacman/pkg), and there isn't support for multiple --cachedir flags (although this would be simple to add)
2. Without a network connection, any cached packages that match the package name are considered the latest version, regardless of version
3. With a network connection, version information is gathered, and if it strictly equals the version of the cached package, downloading and compiling are skipped and the cached package is directly installed, otherwise the package is re-downloaded, re-compiled and cached with the up-to-date version (the old is not removed)
4. All packages are cached (similar to pacman)
5. The default cachedir is populated into the main yaourt script, but could be moved to yaourtrc

[alexoneill]

What can I do to help this along? I think caching would be a great addition!