Return the key on Put calls

This closer mimics the datastore API.
Also check other id type when checking for duplicates

From @mzimmerman
Test for put with incomplete key error
Ignore non / requests, like favicon

In chrome, the test suite was running twice each reload.
Merge branch 'master' of git://github.com/mjibson/goon into self

Conflicts:
	entity.go
	goon.go
	test/goapp/main.go