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 
Just realized you are adding `superagent.js` as bower's main.
i think this should work without `/index`, since `superagent.js` is "compiled" already ?