fixes #47

Fixes #41

Not sure if the method is optimal, but here is some tile-flippin' goodness.

Also add test for parsed image layers.
The test can be also used as an example off how one can get data from
the parsed map.

this just adds a field to the Tile so that data may be parsed and accessed

Still need to parse properties.

The attribute is called "type" but we were previously looking for
"obj_type".

The tmx format supports for objects various attributes. id, gid, name and type were ignored by the parser.
as an result it was not possible not possible to have objects which
are using tiles because of the missing attribute gid.

The enhanced parser is now parsing the additional attributes:
*id  - object id,
*gid - tile no
*name- object name (property)
*type- object type (property)

Added tile id to image, since they don't have to start at 0

This makes TiledError behavior more standard, more expressive, and much
more useful for both library and application developers. Close
mattyhall/rs-tiled #17