Use overriddable environment variable for directory ~/screenly insead of hardcoding
Use overriddable environment variable for git repository instead of hardcoding
Use variable for config dir instead of hardcoding
Use more obvious extension than .bak for modified files
Fixed a minor typo
Allow installer to be run from inside a working copy, without re-cloning from github
Added instructions to README.md on how to install locally, along with what gets changed