Ability to initialize map with bounds only

Currently the directive requires a center to initialize the map, but according to this [link](https://github.com/Leaflet/Leaflet/issues/2068) we should be able to initialize it with bounds only.

I also fixed the bug that bounds were not being set in the scope when loading the map for the first time. (#340)

Now, when you pass only the bounds, the map will have exactly the bounds that you passed, center and zoom will be calculated by leaflet and will be set in the scope.

Protractor tests are passing.
Might be related to issues #340 #335 #300 #295