Merge pull request #5 from adamlazz/master

Timestamp and user key options.
Improve validation of TOKEN and USER vars
Whitespace repair
Config should now be optional

TOKEN and USER may now be specified on the command line, rendering the
config itself optional.
Quote PUSHOVER_URL for additional safety
Rewording usage to flow a little better
Update usage with requirements for TOKEN and USER
Update usage in README.md to match program output