Add an id property to all the elements and implement new methods

[elchininet]

This pull request implements a new id property in all the elements and implement new methods.

1. Add an optional id property to all the elements. If the id is not specified as a property of an instance, a random id is generated.
2. Implement new methods in container classes (as IsometricCanvas and IsometricGroup):
   • getChildByIndex method to return a child taking into account its index
   • getChildById method to return a child taking into account its id
   • removeChildById method to remove a child taking into account its id

[coveralls]

coverage: 100.0%. remained the same
when pulling 9234074 on implement_id_and_new_methods
into bac797d on master.