Merge MDTraj/formats/xtc/ with upstream's xdrfile-1.1.4

I downloaded xdrlib-1.1.4 from the gromacs ftp servers http://www.gromacs.org/Developer_Zone/Programming_Guide/XTC_Library and tried to resolve the diffs between our xtc code and theirs. I also put in a list inside the README that documents the changes s xdrlib-1.1.4. The most important change for this PR is that the gromacs developers changed the license on the xdrfiles to be more permissive, so that removes some ambiguity in our use of them.

I also removed some unused files that we were including, but never using or compiling.