Convert install path to be relative for manpages.

This allows installation in virtualenv environments for regular users.
Prior to this change, a user would have hit a "permission denied" error
similar to:
	creating /usr/local/share/man/man1

	error: could not create '/usr/local/share/man/man1': Permission denied
Merge pull request #2 from henn/virtualenv-fix

Convert install path to be relative for manpages.