Fix Issue #201: When Obj texture map statement contains texture option, assimp will fail to import texture

<ol>
<li> Fix getTexture defect to get texture image url correctly
     <p>According to http://en.wikipedia.org/wiki/Wavefront_.obj_file#Texture_options, Texture map statement can contains various texture option, for example:</p>
	<ul>
        <li>map_Ka -o 1 1 1 some.png
	<li>map_Kd -clamp on some.png
        </ul>
    <p>So we need to parse and skip this option, just keep the last part which is the url of image, otherwise we will get a wrong url like "-clamp on some.png".</p>
<li> Add Clamp option support for obj texture.
   <p>If  texture option "-clamp on" is detected, we will set the texture mode to clamp.</p>
</ol>