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.