Add ASSERT.that(foo).labeled(bar).blahblahblah() to permit re-labelling of awkwardly named subjects (such as true or false)

In the process, also fix up some GWT cruft and improve a few error messages.
label -> customLabel
Merge pull request #78 from cgruber/bettermessages

Add ASSERT.that(foo).labeled(bar).blahblahblah() to permit re-labelling ...