Full django-planet integration.

Including:
 * Installing planet app and its dependencies.
 * Adding planet app urls.
 * Changed navbar url to point to post list
 * Custom templates

This fixes issue #87
Issue #59: Planeta en la portada
Merge pull request #90 from matagus/issue_87

Full planet integration.
Merge pull request #91 from matagus/issue_59

Issue #59: Planeta en la portada
actualizado el requirements.txt