Added TileMap class declaration and linked required libraries.

This new class uses the TMX parser from http://code.google.com/p/tmx-parser/ as
a basis.
Added func to convert hex string to sf::Color.
Added misc functions to rpg namespace.
Added TMX parser code to project.

The code is from http://code.google.com/p/tmx-parser/
Added zlib dependency to README.
Makefile now finds headers and sources itself.
Added basic tile map rendering with SFML.
Fixed bug that prevented recompiling w/o clean.
TmxTileMaps now use sf::RenderTexture.

A RenderTexture containing the tile map is maintained behind the scenes
and can be accessed for easy rendering and manipulation of the tile map.
This should also be more efficient for rendering as RenderTextures are
stored in graphics memory.
Renamed TmxTileMap to SfmlTileMap.
Slightly restructured SfmlTileMap class.

TmxTileMap is passed in on loading rather than being maintained in the
background.
Completely separated SfmlTileMap from TmxTileMap.

SfmlTileMap now works independently of TmxTileMap, so that any changes to the
latter (including deletion) do not affect the former.
Can now change/query layer visibility.
Corrected some method signatures.
Added the ability to modify tiles in memory.