fixed collada import bug + added obj importer '\' newline feature

The collada loaded has a bug, when importing a collada file that contains an animation, then another one containing no animation.
Basically mAnims is not cleared before the second model is being imported, leading to a crash.

Regarding the .obj importer, I got a model where some lines where split in two lines using the '\' to protect the CR. This is part of the .obj format specification, so nothing wrong with that model. I have added some code to handle this case.

(only consider commit #16ae05e, not sure why the other appear here)