GMime as an external dependency

[Bastien Nocera <hadess hadess net>]

Heya,

Yesterday I've replaced the RSS Podcast date parsing in totem-pl-parser
to use GMime instead of evolution-data-server's libcamel, which should
make it easier to build, and be lighter for dependencies.

We require gmime-2.4, which is the current stable version.

Cheers