Lock spinner and add state

This prevents it blowing up if start is called more than once. Start can
now return an error if a running spinner is started.
Add test for multiple spinners
Only allow one spinner.
Merge branch 'master' of https://github.com/briandowns/spinner

Conflicts:
	spinner_test.go
Merge pull request #11 from millere/master

Prevent existence of multiple spinners