Refs #1164. Correction d'un bug dans le paypal frais format.

Signed-off-by: Quentin Avedissian <quentin.avedissian@gmail.com>
Update README.markdown
Documentation.
Cleaned up constants. arrays of constants now contain constants instead of simple strings.
Added a date_format method.
Throws a PayPal_Exception when request couldn't be parsed (which shouldn't happen..).
Merge branch 'refs/heads/3.2/develop' into feature/nvp

Conflicts:
	classes/kohana/request/paypal.php
	classes/kohana/response/paypal.php

Signed-off-by: Guillaume Poirier-Morency <guillaumepoiriermorency@gmail.com>