Added usage information, exit codes, error handling & quote escaping

I added usage information, so if the user does not provide a path to a
plist or provides more than on path they will receive an error message
and a exit code of 1.

I have also added a check to supplied file, if it is invalid than it
will return 1 and it will show an error.

I have also added quote detection to the NSString handler, if it
detects a quote in the string then it will escape it.
Updated usage information
Merge pull request #1 from joshluongo/master

Added usage information, exit codes, error handling & quote escaping