Fixed all issues on v1.04

-Found the bug that made crash the script when the range selected was
before the first frame with a key.
-Now a key is added at the first and last frame of the selected range to
prevent the PC2 merger function to crash.
-This new behavior also makes innecesary the v1.05 that was began to
correct the mesh corruption when an static object is added to the merge
list. Now all the obects are forced to be animated.

To Do:
-The new code to add frames can be highly optimized.