Allow multiple key delete support

The Redis DEL command allows deletion of multiple keys with a single
command.
Updated del() function to support multiple keys as well. Keys can be
passed as an array of strings or as multiple arguments to the function.
Update README

Update README to reflect change where mocking for the DEL command can
now delete mutiple keys.
Code formatting.
Code formatting for code standard
Merge pull request #22 from dchancogne/del-multiple

Add multiple key delete support