ADD bower support

To add this project to [bower](http://bower.io/), you have to perform the following steps:

[Register the package in bower](http://bower.io/#registering-packages):
 * `npm install -g bower`
 * `bower register oauth-signature https://github.com/bettiolo/oauth-signature-js.git`

When releasing a new version:
 * Increment bower.json version as you make in package.json
 * Create a git tag for the version (ex: `git tag 1.1.4` and  `git push --tags` or make a [Github Release](https://github.com/blog/1547-release-your-software)) 

Thats it. No need to publish to bower cause it uses your github tags for versioning