Add support for the path_format xyz layer param, which uses a regex to parse out the full path the files.

If not path_format is given (it's optional), uses the default z/x/y.extension style.

Likely not optimized.
Docs updated to go along with this new option
The makings of a unit test to confirm that this new functionality (path_format) works and doesn't interfer with the existing style.
Merge branch 'test-patch' of https://github.com/jbjonesjr/mapfish-print into xyz_uri_regex
Merge branch 'master' of https://github.com/mapfish/mapfish-print into xyz_uri_regex
Adding the makings/thoughts of how I want to lay out these unit tests
Continue to lay the groundwork for how I want to write the test and verify this url format
Remove dummy tests
update how i do recursive pattern matching (since the pattern is changing as I go through...) and clean up some of the formatting of the tile sizes.
layer spec is now fully functional and the tests pass, proving both former functionality and new functionality exist side by side
Merge pull request #56 from jbjonesjr/xyz_uri_regex

Xyz Tiles supporting arbitrary uri patterns