Import SOFA 20131202

I have run sofa_deriver and imported the resulting code into erfa. This should fix #19

I have updated also README.rst and AC_INIT in configure.ac

What I haven't updated is ERFA_LIB_VERSION_INFO, as it should be done just prior the release. Given the readme of SOFA and the API information in here http://upstream-tracker.org/versions/liberfa.html, there are only API additions, son the final library version should be

ERFA_LIB_VERSION_INFO(2, 0, 1)