Changed import paths to remove unused fmt (compile time error) and requi...

...red full path for go get for base58

Was trying to "go run keys.go base58check.go" to generate keys and ran into two small issues:

* Import path needed to be the full "github.com/prettymuchbryce/hellobitcoin/base58" to run go get (assuming people are using go get to grab github.com/prettymuchbryce/hellobitcoin in the first place)
* Removed unused "fmt" here to fix a compile time error