Added initial implementation of commit graph interface for web-api.
Merge commit '7bd9342ff48b113196c38f3c0f1cf21f808d34d7' into node-test

Conflicts:
	src/web/api/src/main/java/org/geogit/web/api/CommandBuilder.java
	src/web/api/src/main/java/org/geogit/web/api/ResponseWriter.java