Web api changes

Fixes issue #78 Expose feature diff through the web-api. This includes the addition of two web-api commands the getCommitGraph which returns all commits from a given commit including all parents if it is a merge commit and featureDiff which is used by the GeoGit UI in Salamati. It returns the attribute diffs of a feature between two commits. Also updated the Diff web-api to return the needed information for the GeoGit UI in Salamati.