Add color to diffusion blame and improve plain view

Summary:
query the database to get the epoch info for the commits, then
calculate the color depending on the epoch (the newer the commit, the
dark its color). Also improved the plain blame view for git, as the
git-blame doesn't produce a good display by default. Now we format the
output it from the data we fetches from the database.

Test Plan:
verify both git and svn browsing page work for 'plain',
'plainblame', 'highlighted' and 'highlightedblame' view.

Reviewed By: epriestley
Reviewers: epriestley
CC: epriestley, jungejason
Differential Revision: 93