[metrolyrics] fixes: metadata source

As Juan Suarez explained to me on an IRC meeting (22-07-2013) this
metrolyrics 'plugin' is actually a metadata source which should
implement only resolve and may_resolve operations.

In this meeting we stablish others standards such as grilo operations
name will be fix names such as grl_source_search.

This commit implement (and remove) what was discussed.
[lua-library] Fixing util build_media from table.

Now we are using key's name to get the key-id.
Lua plugins can use media.artist = '' and we use 'artist' to get
GRL_METADATA_KEY_ARTIST key and then save the artist value.
[lua-factory] Standard to functions name

Standards such as:
* grilo operations name like grl_source_search;
* source table instead of plugin table

Others to be implemented related to the keys too.
[lua-factory] add supported_operations
[lua-factory] Support to list of keys

lua-factory is saving supported-keys, slow-keys and resolve-keys from
lua-source.
[lua-factory] may_resolve operation