Support registration of JPEG plugins when the classloader can't find sun plugins.

In some situations, the classloader may not be able to find the sun jpeg plugins
even if they are registered.  In this case, we can still find the sun plugins by
iterating over all the registered plugins.
Return immediately when the right sun jpeg plugin is found.
Merge pull request #32 from rtimpe/master

Support registration of JPEG plugins when the classloader can't find sun plugins