Allow configuration of a iceServerGenerator function

This function will return iceServers and allow for regeneration of
ICE Server credentials before each peer connection.
Merge pull request #27 from dugancathal/add-ice-server-generator

Allow configuration of a iceServerGenerator function
Revise implementation to not modify the input configuration - pass {} as first arg to defaults
Use template and just do the first 4 tests
Reword to ignore rather than remove
Bump version to 3.2.0