Added luhn validator.

Added a luhn validator to parsley.extend.js. Uses luhn algorithim to
verify that fields value is correct. Would be used on credit card fields
to assert that they have a valid credit card number, but can be used on
other types of data as well.
Merge branch 'master' of git://github.com/guillaumepotier/Parsley.js into luhn-validator

Conflicts:
	dist/parsley.extend.min.js
	parsley.extend.js
	tests/index.html
	tests/tests.js
Added luhn validator documentation.
Merge pull request #150 from VoiceOfWisdom/luhn-validator

Added luhn validator.