Export the service

[oliversalzburg]

I was wondering why the service wasn't exposed in the published module? I use this project with angular-material and the directive doesn't really work in that environment.

Rolling my own strength indicator from the app directory worked like a charm though. Maybe it would make sense to expose that service for users.

[subarroca]

@intellix has made a pull request with what this. I'll be releasing a new version today I hope. Let me know if you have any problem.