Fix line endings

Currently, some mpmath files use Windows (CRLF) line endings, and some other ones use linux (LF) line endings. We should just let git handle it, the best way seems to always use LF internally, and let `git checkout` to convert line ends to the platform specific ones. The reason is that when you copy & paste some code, the `^M` character pops up in `git diff`, unless you use your platform native line ending.

Some relevant reading:

https://help.github.com/articles/dealing-with-line-endings
http://stackoverflow.com/questions/1889559/git-diff-to-ignore-m

This PR fixes the problem.