DSN and REDIS_BACKEND normalization

[iskandar]

These changes provide more robust support for DSN-style strings.

Before these changes, you could not use a simple DSN like redis://host:port. With them, you can use strings like these for REDIS_BACKEND:

• host
• host:port
• redis://host:port/
• redis://host:port/db
• redis://user:pass@host:port/
• tcp://user:pass@host:port/
• and similar variations

  Notes:

• The 'user' value in the DSN is required even thought it is not used.
• We are really just relying on PHP-native parse_url to do the real work.
• I haven't addressed the idea of using DSN strings for Credis_Cluster instanciation.
• The $database param in the constructor will be overridden by a database value supplied in the DSN.
• This change should be 100% backward-compatible.
• Tests included.
• I fixed some whitespace.

  Please also see:

• https://github.com/chrisboulton/php-resque/pull/181
• https://github.com/chrisboulton/php-resque/pull/162

[chrisboulton]

:thumbsup: Nice job with this!

[iskandar]

:tada: