refactor(bower): require bower instead of spawn

I see no need to spawn bower processes when we can use bower as a node
module. Spawning adds unnecessary overhead and also might be problematic
for people who don't have bower in their PATH, etc.
fix(bower): main path resolving bug fixes

string values (not array) and paths prefixed with ./ were not handled correctly
Merge pull request #15 from shahata/bower

Require bower instead of spawn