docs(README.md): document options for tagging versions

2c75c2a350277aede5d665d264b96d41b7ca43b7 added some new options which allow a bit more control
over the semantics of versioning. This CL adds some documentation.

Closes #45
style(*): remove trailing whitespace

TODO(@caitp): add jshint/jscs rules about this to keep it out of the tree.
fix(grunt-bower-release): failing task if there any issues with git