Implement should_fail

[spookylukey]

@bugaevc This pulls in your should_fail method, with a change (mandatory description parameter, otherwise parse failure messages are just `expected 'should fail'), plus docs.

I'm running out of energy/time to implement other things like many_till etc, but this seems to me to be a good enough building block for now.

[coveralls]

Coverage remained the same at 100.0% when pulling 31d6013 on implement_should_fail into 27e1787 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 31d6013 on implement_should_fail into 27e1787 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 31d6013 on implement_should_fail into 27e1787 on master.