Added matio CHANGELOG

Added CHANGELOG with changes that have been made in matio to build and
work in the rmatio package
Fix Makevars and Makevars.win

MATIO_PLATFORM is defined to "rmatio" when building. The define is not
really used, since rmatio always use the argument hdr_str initialized
from R with the correct platform when calling Mat_CreateVer
Bumped version to 0.6.0