Add Flag Functionality - Issue 470

Adds Flag functionality to Layer and Map pages. 

Requires geonode pull request to add django-flag to shared/requirements.txt  https://github.com/opengeo/geonode/pull/13

Also, required monkey-patching django-flag a bit in order to use the one from pypi. It looks like using the one from the github repo has fixed some of the issues I encountered. Not sure how we handle this in mapstory, but in mainline geonode we always try to use the pypi version whenever possible.