Added commands.

:h          Print usage.
:q          Exit.
:w <FILE>   Print page source to file. Default FILE is output.txt.
:s          Switch sync/async javascript execution.
   exit     Exit.
PEP8 fix.
Merge branch 'master' of https://github.com/askeing/B2G-JS-REPL
Update README.md
Added Async JS execution example.
Merge pull request #2 from askeing/master

Sync