LGInstaller class

This breaks out installer logic from configuration window and puts it into it's own class.
since there was already a LGGitHubJSONLoader to get the latest release array, I built it out to do the same with the git package.

Now AutoPkgr uses the repo https://github.com/timcharper/git_osx_installer.  
This is the same maintainer of http://sourceforge.net/projects/git-osx-installer/ which is where git-scm.com directs you to.

The other enhancement is that the LGDefaults actually set the GIT_PATH in com.github.autopkg so now it gets hardcoded for autopkg itself and doesn't require AutoPkgrTask to set the path variable in the environment. 

Fixes issue #61