Removed the BUILD_CLAR CMake flag, and updated the readme.

Add unit tests to confirm ignore directory pattern matches and
to confirm that ignore and attribute files are loaded properly
into the attribute file cache.

Per issue #533, the handling of relative paths in attribute
and ignore files was not right.  Fixed this by pre-joining
the relative path of the attribute/ignore file onto the match
string when a full path match is required.

Unfortunately, fixing this required a bit more code than I
would have liked because I had to juggle things around so that
the fnmatch parser would have sufficient information to prepend
the relative path when it was needed.