nsqlookupd: refactor daemon to be importable

Based on discussion here: https://groups.google.com/forum/#!topic/nsq-users/5OTGtodB9Co and here: https://twitter.com/dinedal/status/421025449930534912 it appears that a preview of what a refactor of NSQd and NSQLookupd to importable package structures would look like is needed.

Since there are two programs to apply this to, and since NSQLookupd is the easier one to refactor, I started on NSQLookupd.

Due to go's importing, if you want to test this, you must change main.go:7 from:

```go
    "github.com/bitly/nsq/nsqlookupd/imports"
```

to:

```go
    "github.com/dinedal/nsq/nsqlookupd/imports"
```

Please let me know what you think, I took the approach of exposing the minimum required functions and structs that were not already exposed.