Add state accessors like `self.pending`

Closes #21
Accept all 2xx codes, not *just* 200

/cc @tunam3lt
Don’t throw if user attempts to reseal a Promise 

Closes #25
[Promise value]
Test promises with void return don’t break when

Refs #27
Fixes mismatched selector types
Merge pull request #29 from krzysztofzablocki/master

Fixes mismatched selector types
Merge remote-tracking branch 'upstream/master'