Make realpath.c available wherever hermit goes

Code taken from
http://stackoverflow.com/questions/284662/how-do-you-normalize-a-file-path-in-bash
Use realpath in hermit add to make subdirs relative paths work

Fixes #57.  The good thing is that add and update are the only
subcommands that really need to care about this, since link and unlink
don't make assumptions about the current directory and use absolute path
always.