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.

I tried to make this as unobtrusive as possible for this PR.  As discussed [here](https://github.com/phuesler/ain/pull/21), we realized that forcing unix_dgram as a required dependency will make it difficult for machines that do not have the capability to build the associated module with node-gyp.

Let me know if I can make some adjustments to the PR.