Allow custom handling of multipart data.

Multipart is often used to upload files or large chunks of data.
It is not always desired to have this stored in memory, or even
on disk (for instance if processing can be done during the data
is transferred). Add two custom handlers for the multipart parts.
Simplified logic
BodyParser: updated documentation.

The body parser documentation have been updated with all
the valid options and a short description of the impact
on the body parsers behavior they have.
Added tests

Tests for both normal multipart/form-data, which was missing
and for the custom handlers.
Merge pull request #548 from noseglid/master

Allow custom handling of multipart data.