Add a empty alias for noContent

[Taluu]

I'm not sure how to deprecate things, but I "deprecated" noContent and transformed it into an alias for the new empty assertion.

Fixes #249

[rquadling]

I'd leave out the @deprecated for the time being. Deprecating an assertion can cause end users a real nightmare if there is not a clear path an a LOT of notice.

[Taluu]

I removed the @deprecated. I thought that the given message (using empty instead) would do the trick, unless you'd like to add a trigger_error with it ? Not sure what would be a better deprecation path though...

[Taluu]

Rebased and updated from upstream/master.