Move fbx-conv to tools [skip appveyor][skip travis]

tools/fbx-conv/README.md

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+fbxconverter version 0.7
+It is for cocos2d-x after version v3.3
+
+Convert .fbx to .c3t used by cocos2d-x. Thanks to libgdx fbx-conv. We use their code and do some change.
+
+You can get the source code from the address below:
+https://github.com/cocos2d-x/fbx-conv
+
+USAGE:
+
+Open a command line and enter into the fbx-conv directory, and then input the full path fbx-conv file (you can drag fbx-conv to command line directly) and then enter the parameters -a (-a means export both of c3t and c3b file, if parameter is empty we export c3b file by default), and then enter the file name which you want to convert.
+
+Command line:
+cd [path of fbx-conv]
+xx/xx/fbx-conv [options] <input>
+
+For example:
+1. cd /Users/xx/cocos2d-x-3rd-party-libs-bin/fbx-conv/mac
+2. /Users/xx/cocos2d-x-3rd-party-libs-bin/fbx-conv/mac/fbx-conv -a boss.FBX
+
+You can use the -? option to see more help information.
+
+Note: 
+
+1. Model need to have a material that contain one texture in the diffuse-channel at least.
+	
+2. About animation, we just support skeletal animation.
+	
+3. Just one skeletal animation object and no support multiple skeleton.
+	
+4. You can export multiple static model , so you can support a static scene.
+	
+5. The maximum amount of vertices or indices a mesh should less than 32767.