Improve non-required fields situation

This remove ``omitempty`` from named struct field declaration (#7), and take in account if the fields that are required or not when generating function options.
It also add some helper function ``Bool``, ``Int64``, ``Float64`` and ``String`` to deal with pointers more easily.

And example output can be found here: https://github.com/cyberdelia/heroku-go/blob/master/v3/heroku.go