Was not working with latest grunt rc, refactored and brought up to date

[jsoverson]

• Updated readme
• Updated for grunt 0.4.0rc7 apis
• Fail on errors
• FIxed tests
• Added non-casper test tasks
• Changes configuration api to pass dest as --save
• Warn on invalid options in non-test mode

[jsoverson]

Also added gruntplugin keyword so this task can be discoverable on gruntjs.com next time it is published to npm.