magnatune: Adding Magnatune plugin

Search implemented for Track title, Albums and Artists names.
The function looks into database provided by magnatune website.

A subset of keys are returned and could be incresead when implementing
other functions.

https://bugzilla.gnome.org/show_bug.cgi?id=698523
magnatune: Adding support to download database

All database and crc checks are made when user interact with the plugin.
If there is no database and no internet connection, plugin_init fails.
When current database has 7 days old the plugin starts to check for a
newer database.
To avoid checking crc to *every* user request a time interval of 12
hours is used.

https://bugzilla.gnome.org/show_bug.cgi?id=698523
magnatune: Implementing browse operation

Application can browse throw Artists, Albums and Genres.

https://bugzilla.gnome.org/show_bug.cgi?id=698523
magnatune: add C file for translation

Include grl-magnatune.c in POTFILES.in