magnatune: Do not check for network connectivity

It fixex huge delay resolving dns.

g_network_monitor_can_reach() is sync and takes up to 30 seconds to
resolve dns with some users.

Link: https://bugzilla.gnome.org/show_bug.cgi?id=701336
tests: local-metadata: Add test

Tests if thumbnail can be obtained from local content.
tmdb: plug some leaks
[metrolyrics] First version of this lua plugin

Searching and Browsing are working. There is a test for search in the
end of the file, you can remove the comments and run the plugin to get a
lyric from metrolyrics.

Browsing has some things to be done yet, as getting next pages of
artist or when browsing in alphabetic order.

Should be done when lua-factory passes the value of 'count' and 'skip'.
[lua-factory] Add folder to the build settings
[lua-factory & lua-library] Initial functions

* lua-factory: Load lua pluggins (for now, only from enviroment variable
  GRL_LUA_PLUGINS_PATH); Initialize each lua stack and load lua-library
  (as grilo module) to them.

* lua-library or grilo module to lua: has two functions:
  - callback: sent media to the user
  - fetch: get web content from GrlNetWc (Async!)
[lua-metrolyrics] organized with factory/library

* The search in previous commit was in fact a query
  (by artist and music); Metrolyrics has a search function
  which is being used by current commit;

* Search, Browse and Query will most-likely return the metadata
  information and the user should do Resolve to get the lyric;