implement gcfg

This ended up being a lot simpler than I was expecting. Now we read from a config file into a struct and use those values.
I think we should follow [Google's import guidelines](https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Imports) and format imports like this:
```go
import (
    "fmt" // stdlib

    "github.com/NebulousLabs/Sia/siafiles" // our project

    "code.google.com/p/gcfg" // external project
    "github.com/spf13/cobra" // external project
)
```