Use Time#xmlschema instead of reimplementing it in custom method
wsu:Created and wsu:Expires should always be in utc format

"All times MUST be in UTC format as specified by the XML Schema type
(dateTime)" - excerpt from chapter 10 Security Timestamps

http://docs.oasis-open.org/wss/v1.1/wss-v1.1-spec-errata-os-SOAPMessageSecurity.htm
Fixes issues with WS-Security digest passwords

wsu:Created and wsu:Expires should always be in utc format
fixes timestamp accessor
Fixes nonce decoding problem with Username Token authentication
clears the nonce after each use
merged jessbowers/akami

conflicts:
	lib/akami/wsse.rb
updated development dependencies
updated urls
updated the changelog [ci skip]