pm : added bower.json

Adding a manifest file for package mangement via Bower (simplefocus/FlowType.JS#4)

Might want to change the version tag to your chosen value (be sure to use semver) and add a corresponding tag on the tree. Then run `bower register Flowtype.js git://github.com/simplefocus/FlowType.JS.git` from the command line to register the package.

Should keep them happy :)