Bug fix for animation frame loading

Previously, the code was passing each <animation> tag (of which there
is, at most, one) to the TmxAnimationFrame constructor.  Now, it will
pass the <frame> tags to that constructor.  This fixes a bug where a
null reference exception was thrown whenever loading a .TMX file which
uses animations.
Tileset tiles: added the "Gid" (global id) property

Trying something new... this might be convenient/useful, or it might end
up being unnecessary.
Revert "Tileset tiles: added the "Gid" (global id) property"

This reverts commit e7efd4be829284841d6a0151e81948d7ada93bcf.
Merge pull request #8 from Josh1billion/master

Bug fix for animation frame loading