Change in default behaviour to ignore symlinks and hardlinks

During large hashing operations of data such as a system installation,
symlinks may be encountered. When it is copied data from another system,
the symlinks will probably fail and create significant noise in the hashing.
I don't care at all about symlinks, so have changed the script to ignore
such links by default (unless they're explicitly referenced in a checksum
file). This behaviour is configurable by the new '-l/--links' option.
pylint pass
Version update: v0.10