Support the host/currentHost options to defaults

`defaults` allows either `-currentHost` or `-host [hostname]` to be
given as the first argument. This is required to set certain system-wide
values on OSX, e.g. NSGlobalDomain com.apple.mouse.tapBehavior.
Add simple specs around boxen::osx_defaults
Ensure the 'defaults' command is actually correct
Support Ruby 1.8.x
Test the '-host' parameter
Fix the value check when a host is given
Merge pull request #18 from dgoodlad/support-host-option-for-osx-defaults

Support -host/-currentHost options to defaults