Load Tilesets From External Files

Fix Map::get_tileset_by_gid

Add keywords metadata to Cargo.toml

Add properties to Objects and parse properties as enums

Still need to parse properties.

Because I forgot. Again. :(

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

If I don't forget to publish the changes then I forget to update the README. *sigh*

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

Add Error trait implementation for TiledError.

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

Make compatible with latest version of Rust

Edited orientation keywords to be compatible with Tiled 0.14.2

- Added the orientation keyword 'hexagonal' which is used by Tiled 0.14.2

- Changed the orientation keyword from 'Staggered' to 'staggered'. This is
  consistent with other keywords and it is also the one used by tiled
  0.14.2.

Update for rust v1.4

 * change readme build button back since this will be a pull request

Fix dependencies to make the main library compile