Updated bower.json to valid format

Running “bower install epoch” from a non-cached version of the package
threw the following warnings:

bower epoch#*               not-cached
git://github.com/fastly/epoch.git#*
bower epoch#*                  resolve
git://github.com/fastly/epoch.git#*
bower epoch#*                 download
https://github.com/fastly/epoch/archive/0.5.2.tar.gz
bower epoch#*                  extract archive.tar.gz
bower epoch#*             invalid-meta epoch is missing "main" entry in
bower.json
bower epoch#*             invalid-meta epoch is missing "ignore" entry
in bower.json
bower epoch#*                 resolved
git://github.com/fastly/epoch.git#0.5.2
bower epoch#~0.5.2             install epoch#0.5.2

I updated the bower.json file to get rid of these warnings. I also
added some keywords to improve searchability on bower package search
engines.