[go] Add word-count exercise
[go] Add basic WordCount function signature
[go] Implement WordCount function w/ passing tests

I like how Go's Map type starts with default values that are relevant to
the type:

```go
m := make(map[string]int)
m["foo"] == 0

s := make(map[string]string)
s["bar"] == ""
```

I do *not* care for Go's lack of functional primitives like `map`.
Writing a for loop to append to some slice feels rough. Perhaps I'm
missing something.

Submission: http://exercism.io/submissions/9b23000be2c7b44f8eefbfad