#20: Added MustParse function for versions (thanks @adamreese)
#15: Added increment methods on versions (thanks @mh-cbon)
Fixed
Issue #21: Per the SemVer spec (section 9) a pre-release is unstable and
might not satisfy the intended compatibility. The change here ignores pre-releases
on constraint checks (e.g., ~ or ^) when a pre-release is not part of the
constraint. For example, ^1.2.3 will ignore pre-releases while
^1.2.3-alpha will include them.
