Merge pull request #11 from tlivings/master

Makes apiPath required
Use 501 instead of 500 as default HTTP code

501 is "Not implemented" while 500 is "Internal server error".
"Not implemented" seems more realistic for a stub.
Merge pull request #13 from sgmap/501

Use 501 instead of 500 as default HTTP code
Correct response property

`server.inject` response callback exposes the response contents in the `payload` property
Merge pull request #14 from MattiSG/patch-1

Correct response property
bumped version