Add tests

Add a rspec test suite covering nearly everything (only the ```Fleet::Controller``` private methods are not tested).

I also have made a couple of fix in the code:
- ```Fleet::Controller```: define ```#start```, ```#submit``` and ```#load``` together (metaprogramming)
- do not rely on some active model methods and rails load path
- Use hash instead of named parameters in constructor (would crash with newer version of fleet which return more parameters than used)
- fix the Discovery module access to hosts (via discovery_url)

Thanks for creating this library and I hope this PR will help.