fixed #1662: Getting full path will fail if texture name in the plist is without relative path.

For example:
<pre>
Resources-
         |
         Particles-
                  |
                  a.plist(texture name is `a.png`, just a file name without relative path)
                  a.png
</pre>
a.plist and a.png are at the same directory. 
When `CCParticleSystem::initWithDictionary` is invoked, the second parameter `dirname` will be `Particles/`.
The `Particles/` will not be inserted before texture name.