Frame basis pair

Clarified a few comments according to https://github.com/mockingbirdnest/Principia/issues/139#issuecomment-54760799.
Renamed `ICRFJ2000EclipticFrame` to `ICRFJ2000Ecliptic` since
- ICRF stands for International Celestial Reference Frame, so that sounded like PDF format,
- This represents the (reference frame, basis) pair (ICRF, [something based on ecliptic at J2000]), not just a reference frame.