Added a new lyrics provider

With these changets, I've added support in glyr for another lyrics provider, [Vagalume](http://www.vagalume.com.br/api/docs/). It's a Brazilian lyrics provider that happened to have lyrics that glyr couldn't find in any other (for example, A.C.T - Manipulator).

I was using cmus-lyrics to display the lyrics for running songs and sometimes a really random song was returned, which led me to add this provider.

Their API is JSON-based and I've included in this pull request code to convert from unicode escape sequences into UTF-8 text using regular Glib functions (specially since with Portuguese text non-ascii input is a _must_). Also, I've set a fairly high quality and speed values for the provider (90), if you think that's too high please let me know.

In my system, I have Glib 2.34 and I'm not sure about where to look if all of the functions I've used are available since 2.10 (the version requested in the CMakeLists file). Do you have any tips about where to check for these kind of things?

About the code itself, I've tried to follow your code conventions, but if you feel that something is out of place, please say so and I'll fix it.