Enable on demand unloading

[rhaschke]

Fixes #37, issue1.cpp. Relates to #38, which is fixed by ros/class_loader#34.

[rhaschke]

The problem is definitely in libclass_loader only, which requires that the ClassLoaders stay in place as long as objects do.

[rhaschke]

With ros/class_loader#34 the unit tests should succeed and on-demand loading/unloading should work again.

[mikaelarguedas]

Hi @rhaschke
Given that ros/class_loader#34 does fix the library loading issue for ondemand loading/unloading but doesn't solve the unloading problem when the ClassLoader is destroyed;
Do you think that enabling ondemand loading/unloading in the pluginlib's ClassLoader still makes sense ? It will result in the same SEVERE WARNING and not unloading the library when ClassLoader destructor is called. As a result #37 issue1.cpp will not be fixed by this PR. Is that right?

[rhaschke]

@mikaelarguedas
Indeed, one gets the SEVERE WARNING when trying to destroy the ClassLoader when objects are still in use. Hence ros/class_loader#34 doesn't fix both issues of #37.
I was thinking about to drop PR #39 too, because there is not benefit yet in using on-demand unloading of libraries: Either there is the SEVER WARNING or a segfault on exit of the program...
Hence, feel free to close this PR without merging.

[mikaelarguedas]

@rhaschke I will then close this PR. The related issue can be kept open if anyone wants to take over and offer a solution to fix them.
Thanks for your work,