Stopped parsing dates using regular expressions, and instead used Ruby's
own iso8601 parsing method

The initial reason for this was that date-time should allow 'YYYY-MM-DD'
according to iso8601. Ruby allows this but schema-form did not.
Stopped all formats requiring uri

I assume that's a hangover from when they were all in the same file as
the uri format
Merge pull request #118 from iainbeeston/ruby_validation_of_dates

Conflicts:
	lib/json-schema/attributes/formats/ip4.rb
	lib/json-schema/attributes/formats/ip6.rb