Added bower.json

The changes in this commit make the module installable using bower.

The change to client.js is backwards compatible with `component build` but also enables installation with bower + browserify + debowerify (the caveat is that one must use `require('superagent/lib/client')` instead of `require('superagent')`)

Addresses #315 
this breaks compatibility with component/duo i think.
IMHO bower should consider adding `index.js` to it's main list, like any other pkg manager.