Get rid of "ipaddr" module from requires

Because of

* ipaddr is used to validate whether given value is IP Address
* `hostname` parameter in config may be hostanme or IP Address
* "python-ipaddr" RPM doesn't exist in AWS RPM repository
Merge pull request #16 from JumpeiArashi/get-rid-of-ipaddr

Get rid of "ipaddr" module from requires
Support foreground option '-f'

This option is useful for running blackbird under like systemd or supervisord, etc.
Merge pull request #17 from makocchi-git/support_foreground

Support foreground option '-f'