added .gitignore, now *.asv files are not commited

Everybody has .gitignore now. To remove *.asv files that are already
tracked by git, just tell git in a shell:
git rm --cached *.asv
Then no new or old .asv files will be commited in the future.
Merge branch 'master' of https://github.com/mor1pheus/HybridTracking