Skip failling tests due to recent github handling modification of include-tag

Github recently (25th of december, 2013) changed its handling of the include-tag option. Skipping tests while waiting for a proper solution to be found.

Same thing as libgit2/libgit2#2020
Delegate Commit.MessageShort to libgit2 (#593)
Prefer git_patch_print to git_diff_print

This has the nice side effect of allowing binary changes
to be detected.
Update error categories to match libgit2
Deploy Proxy.MarshalAs<T> more extensively
Introduce marshaling extension to IntPtr
Make the object database use posix-style paths.

Modify a test case to check whether the object database accepts paths that
are not valid on Windows.

Add in that same fixture a test of TreeDefinition.Remove .

Replace instances of Path.Combine by FilePath.CombineGitPaths() .

Change instances of FilePath to instances of string in the object
database, add some tests, add a static method to concatenate git paths.

Update methods documentation to explicitly say that the paths to use are
relative posix-style paths in TreeDefinition, Commit and Tree.
Merge branch 'PosixStylePathsForOdb' of https://github.com/clementbellot/libgit2sharp into PosixStylePathsForOdb