ENYO-2565: download PhoneGap-built packages (+ refactoring)

- `bdPhoneGap#returnBody()` becomes `bdBase#returnFormData()`
- `returnFormData()` fix global variable leaks (bug root-cause)
- `returnFormData()` can deal with multiple parts
- `returnFormData()` can handle both Streams and Buffers
- `returnFormData()` place-holder code for node-0.10.x (switch from
  `Buffer` to `Stream`)

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>