Deprecate w3lib.form.encode_multipart. Fix GH-14.
Remove w3lib.form from README; add a warning to encode_multipart docstring.

Deprecation warnings are silenced by default in Python 2.7, so it is better to add a warning to the docstring as well.
Merge pull request #17 from kmike/deprecate-w3lib-form

[MRG] Deprecate w3lib.form.encode_multipart. Fix GH-14.