Rearchitecture of grimoirelib_alch and misc details

grimoirelib_alch has been heavily restructured. It is now a package structured in subpackages, and the classes hierarchies have also gone thorugh deep restructuring.
Analysis for both demographics (attraction / retention) and timezones are now complete (including scm, its, mls for the former, scm and mls for the latter).
New docs directory, with the needing files to use sphinx to generate documentation (see http://jgbarah.github.io/GrimoireLib for the result).
Minor changes here and there. However, only grimoire_lib and docs dirs, and the files vizgrimoire/analysis/ages.py and vizgrimoire/analysis/timezone.py are affected.
This pull request includes all funcitionality in #36, which I'm closing in a moment, as superseded by this one.