OPEN - task 3: Index-Maps 
http://github.com/markus1978/emf-fragments/issues/issue/3

- added functionality to remove objects from an indexed map
- added test cases for iterators and remove from indexed maps
OPEN - task 3: Index-Maps 
http://github.com/markus1978/emf-fragments/issues/issue/3

- added tests for indexed maps to the base test suite.
OPEN - task 3: Index-Maps 
http://github.com/markus1978/emf-fragments/issues/issue/3

- added tests for the `next` function of indexed maps.
OPEN - task 3: Index-Maps 
http://github.com/markus1978/emf-fragments/issues/issue/3

- fixed a small bug in the last commit.
OPEN - task 3: Index-Maps 
http://github.com/markus1978/emf-fragments/issues/issue/3

Added some more indexed map tests.
Switched the extrinsic ID index for cross references from String type to
Long type, since the Long provides a more intuitive computation of the
next key.