[FIX] "UnboundLocalError: local variable 'mtime' " when with --watch

If a file is not compilable on the 1st try, `mtime = os.stat(filename).st_mtime` would not ran, so `mtime` will not exist on `if st.st_mtime > mtime:`
Credit  #33
Merge pull request #32 from alanjds/patch-1

[FIX] "UnboundLocalError: local variable 'mtime' " when with --watch
Include b64 headers
Fix broken build on Mac