Support connecting with a unix domain socket

[Ramblurr]

As discussed in #285

This changes the semantics of --server (or the env var, etc) to --server <IP/FQDN | /path/to/socket>

[deitch]

Thanks again for putting this in.

A few comments:

1. The docs should be updated, specifically the main README and docs/configuration.md
2. We should have some kind of test for this, even just a unit test of Connection.MySQL()
3. Should --port be deprecated, and everything move into --server? So you can do --server 127.0.01:3306 or --server 127.0.0.1 or --server /path/to/unix, and `--port will be ignored?

[Ramblurr]

1. The docs should be updated, specifically the main README and `docs/configuration.md`

Done!

2. We should have some kind of test for this, even just a unit test of `Connection.MySQL()`

I'm afraid I'm not much a go-lang user myself so I don't feel comfortable adding this, would you be able to add the test? (If there was already a similar test I could modify it).

3. Should `--port` be deprecated, and everything move into `--server`? So you can do `--server 127.0.01:3306` or `--server 127.0.0.1` or `--server /path/to/unix`, and `--port will be ignored?

I don't think that's necessary. Port is already optional, and this change preserves that. Since the go-sql-driver does accept Addr as host:port, it could be done.

I leave that decision up to you as the maintainer.

[Ramblurr]

I've enabled the "Allow edits by maintainers" feature so feel free to push to my branch if you want.

[deitch]

This looks good to me. I won't be able to add enough for the testing for the next week at least, and don't want to hold it up. Existing testing should catch any errors for current functionality, and unix-domain socket is new, so I am good with that.

I approved it. Anything else you want before we merge it in?

[Ramblurr]

Existing testing should catch any errors for current functionality, and unix-domain socket is new, so I am good with that.

I did run a local manual test to ensure that I didn't break the tcp connect functionality.. both unix and tcp are working as expected.

I approved it. Anything else you want before we merge it in?

Nope, nothing for this commit. Good to merge.

I've noticed a few documentation inconsistencies that I think are a result of the bash -> golang switch, but I'll open another issue for those.