Libgit2 version

Hello,

It would be nice to get access to the version information of the libgit2 library in use.

As that information won't change, I think it makes sense to add this information as constants instead of providing access to the git_libgit2_version() function.

In fact I started by looking in the libgit2 documentation and found only git_libgit2_version() there, but while writing this pull request, I realized that git_libgit2_version() simply returns the values of the LIBGIT2_VER_* defines which are public, so I think it makes sense to use the latter directly. I updated the branch accordingly (64ae9e1). 

Speaking of version information, pygit2/version.py says '0.17.3' even though the current master is not compatible in places with the released 0.17.3. Wouldn't it be better to upgrade that number plus a dev suffix right after a release? (so 0.17.4dev for now).