Fix 'git-crypt init' for newer versions of Git

At some point between Git 1.7.1 and Git 1.8.1.3, both 'git reset' and
'git status' stopped noticing that files were modified after their
smudge filter changed.  Consequentially, 'git reset --hard HEAD' would
not decrypt existing encrypted files in the repo.

This commit changes 'git-crypt init' to use 'git checkout -f HEAD
/top/of/repo' instead, which does the job.
Escape arguments to filter commands

This will allow both the path to git-crypt and the path to the key file
to contain arbitrary characters, notably spaces.
Enhance documentation

Add NEWS file, spin installation instructions into INSTALL file, and add
useful information from the git-crypt website to the README.