Correctly round-trip attachments (#9, #10)

The primary change introduced by this PR is to remove the base64 encoding around attachment serialization. Whilst this could be viewed as a breaking change (attachments already stored by MyCouch will already be incorrectly base64 encoded), I think this is really a bug fix. For any users who are affected by this bug, they would have to manually decode the base64 encoded attachment before using it.

The other 2 parts of this PR:
 * Add an http request factory for attachment GET/PUT/DELETE requests. I've added a single class to handle all three which generics makes a little messy so welcome suggestions as to whether a reflection-based approach would be better.
 * Send and parse Content-Type headers for attachment requests (#9).