Added two example maps

The sewer example shows the use of a transparent color in the tileset,
while the desert example shows the use of tile margin and spacing.

examples/desert.tmx

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE map SYSTEM "http://mapeditor.org/dtd/1.0/map.dtd">
+<map version="1.0" orientation="orthogonal" width="40" height="40" tilewidth="32" tileheight="32">
+ <tileset firstgid="1" name="Desert" tilewidth="32" tileheight="32" spacing="1" margin="1">
+  <image source="tmw_desert_spacing.png"/>
+ </tileset>
+ <layer name="Ground" width="40" height="40">
+  <data encoding="base64" compression="gzip">
+   H4sIAAAAAAAAA+2Y2QrCMBBFo31wA+sCrlirdd/q/3+dIzZUhtAmMRlH6cPBBlJyvDO2g5EQIvJIFwiBnudzbLgp/JYMvH4lv2dWI2AMTBj4qKgBdSBA3j7OmgIzA+ZAC2gDHYIsVsDagMSj36HAbwOcgO0X/YrySzTzpPDbMfS7Iy/sdxGv/r8S++Gew+uyvHBfuvBT/Q4OKD95XeaHc3WV3x6tpY9pfr7riz0xx6x+upwd+DUymiJ/FxURaO6TLD70C0U+X8jvWkRHc5/qPhu/scjniz4wKGGouU91n8/eoyYW6jmHk2dM6GMzm1F43bJPnWfn+/sncXS+Ljp+T6d7tpfSL2Xoh/uam59Nfd/nCyq/1MAPZ8ktP1WtOftR5mczm1H6/QsxA4dfAWfF7b/BqsYVFZ/xABSXR5cAGQAA
+  </data>
+ </layer>
+</map>

examples/sewers.tmx

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE map SYSTEM "http://mapeditor.org/dtd/1.0/map.dtd">
+<map version="1.0" orientation="orthogonal" width="50" height="50" tilewidth="24" tileheight="24">
+ <tileset firstgid="1" name="sewer_tileset" tilewidth="24" tileheight="24">
+  <image source="sewer_tileset.png" trans="#ff00ff"/>
+ </tileset>
+ <layer name="Bottom" width="50" height="50">
+  <data encoding="base64" compression="gzip">
+   H4sIAAAAAAAAA+3X2QrCMBAF0OKbSwWrCC4vdV/+w///JKeQgWFIatJ0SfE+HJDGonfaTJI8y7IlORkLkotrZ3I0SjUux2zfCSX/h/7tX/T9/5jjQl7kalSfb4nk2JAJ2Y78eUzJjMwjcnAtQnHt2sixIgVZR+TgWtjca8a4dvy+viNyaE1ycC2kh+WaZqvdUDmeppYxQp5DV313KG3n2JE9OYw8R2m5rw+638WuHzw/mrzjMWS/81k/ZM6U5ofsdz7rh8yZ0vxw9VtX361yfkzOlOZHSC/Xa4NtDgw1PwAgDvdSre/eGot7aV1PHgPbWTW1/a5vDn1WTW1f4ptDn1Vd+5KUyf1IkdXvS1LmOqti7wEAAAAAAAAAAF37AlFWzCEQJwAA
+  </data>
+ </layer>
+ <layer name="Top" width="50" height="50" opacity="0.49">
+  <data encoding="base64" compression="gzip">
+   H4sIAAAAAAAAA+3WOwqAMBBAQQu1Uq9i4a8QO+9/JrcwF5BADMzAI92y6bZp/mUvvUAm/vEvtf5jjpa3NdrKrvPZEZ3R/b5Xprlt1EV9pnmlDNEYTaUXAQCgCukOTtV6D6c7OOUeBgAAAAAghwe55M5JECcAAA==
+  </data>
+ </layer>
+</map>