Changed boolean checks to a Is*() naming convention.

Issue #35

This changes the boolean Check*() functions to the Is*() convention that is more common in the standard go libs.

[CheckResponse()](https://github.com/google/go-github/blob/master/github/github.go#L326) was not changed since this is not a function that returns a boolean value. I'm fine with leaving it this way, but I'm open to discussion to change it.