<Request builder>: Support for PostForm

Added support for PostForm(). Essentially a wrapper around POST()
and adds the correct header. WithFormValues() then adds key-values
to the body.

Fixes: #2

author: ZachPlunkett <[email protected]>

Support for Request Context

* Added new API to request builder WithRequestContext(context.Context)
* Update to Documentation

Fixes: #9

Updated Readme.

Minor Changes to Readme