Add .patch version getter

[szszszsz]

Add getter for the .patch part (as in x.y.patch) library version.

Related #35

[robinkrahl]

Two suggestions regarding version handling:

• If you have to manually set the major, minor and patch version in version.cc anyway, we can remove the code to automatically generate them.
• Regarding the version string: Currently, it is hard to tell whether this string will contain useful information. Therefore, my suggestion is to either completely remove the string, or to leave it empty if libnitrokey is not built from the Git repository (so that clients can detect whether it contains useful information).