abstract out TileRangeMask interface from RasterMask, allowing other masking approaches
Merge pull request #238 from groldan/tilerange_mask

Abstract out the interface of RasterMask to allow for other kinds of discontinuous tile ranges
Fix a bug by which truncate tasks are aborted.

FilePathFilter was accepting 'gridset' directories siblings of the one
it's supposed to evaluate as if they were intermediate directories
indide the gridset's tiles layout, which caused a NumberFormatException
in FilePathUtils.findZoomLevel() as it was called with a gridsetprefix
that didn't match the one being evaluated (i.e. the sibling ones)
Make TileLayer.getMetadataURLs() concrete, subclasses should override.

The method was introduced in the patch 1.5.1 version but its
an API change that breaks upgrading other projects to 1.5.1+

Making it concrete with the default and expected return value of null,
preserves the API compatibility.
update site location in RELEASE_GUIDE
Release notes for 1.5.2