Added path as configurable property, unix_dgram optional dependency.

When path is specified, it will check for whether unix_dgram is
installed.  If not, it will throw an error.

When path is not specified, it should just behave as before, including
using node's dgram module.
must add unix-dgram as an explicit optional dependency
Merge pull request #22 from badunk/unix-dgram

Added path as configurable property, unix_dgram optional dependency.