Submodules

Added use of git submodule to integrate the dependency repositories for ADCore, ADBinaries and ADProsilica into the top-level areaDetector repository.

The use of submodules allow us to clone the top-level repository with the --recursive flag in order to download (clone) all of the submodule dependencies as well. The submodules are defined as a repository URL and a revision number.

A wiki page [Using git submodules](https://github.com/areaDetector/areaDetector/wiki/Using-Git-submodules) has been added to describe how to use this feature of git.