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

Piston Renderer example

modified objects to take floats instead of integers

Fix doc comment

Update for warnings

This commit also fixes a bug that meant the last cell of each row would
not be parsed if the tmx file was base64 encoded.

Enable a feature and use split instead of splitn, whose meaning has
changed.

Fix for "private type in exported type signature"

|&:p| is no longer valid syntax. It is inferred.

Update to rust nightly