Started to implement a form callback to use the form-data module
Making sure destination and source are correctly mapped together.
Adding tests
Multipart/form-data implementation

Added the ability to alter the FormData object directly when submitting
multipart/form-data. There is now a callback function option that can
replace the `form` option.

See #8
Updated README

See #8
Merge branch 'form-callback' of github.com:johngeorgewright/grunt-http into form-callback
Merge pull request #9 from johngeorgewright/form-callback

Form callback