updated assembleRegion() in region_growing_rgb.hpp to speed up algorithm

the original code perform std::vector<>::erase() in a while loop. since std::vector<> is an implementation of array, this operation moves big chunk of data at each iteration, and thus dramatically slows down the algorithm. 

Instead I copy all data into a std::list<>, perform erase() on it and then copy data back into the original std::vector<>. It's not a very neat tweak but still much faster.

Tested on region_growing_rgb_tutorial.pcd and other point clouds, outputs are identical to original code.

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![test_result](https://f.cloud.github.com/assets/4807473/2299532/7693dcce-a0ce-11e3-871e-46a05fca5d20.png)
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![test_result2](https://f.cloud.github.com/assets/4807473/2299555/d2cf3c0e-a0ce-11e3-926f-f1e155868c4f.png)