Add POST form data (from NSDictionary) support

Converts a NSDictionary containing key-value pairs into a string, using its data representation to set as an in-memory upload through setUploadData:withContentType:

Also adds BBHTTPURLEncode utility function to convert a string into its URL-encoded equivalent.

Closes #16
Cosmetics

Because apparently Xcode thinks the URL ends with ")"...
Add unit tests

* Fixes bug where setUploadData:withContentType: always returned NO
* Assumes all data coming in for form data (keys and values) are NSString