Fix #140 (Chmod on folder throws error)
Fix #139 (Return error on invalid chmod).
Merge pull request #141 from PressLabs/fix-chmod-on-dirs

Fix chmod (restrict modes to 0755, 0644 on files and 0755 on folders).
Update README.md
Merge branch 'master' into update-keep-policy
Remove redundant error methods
Change the name of the file to keep (from .__keep__gitfs__ to .keep) and update the mkdir method to bypass the create method.