Fix an issue that GPUImageMovie instance is retained by block object but not released

Recently I ran into memory warnings and at last crash after playing many GPUImageMovie objects.

By doing profiling in Instruments (referring to http://stackoverflow.com/a/12286739/1241690), I found that in GPUImageMovie.m, "self" is implicitly retained by the block object in "startProcessing" method.

According to the "Use Lifetime Qualifiers to Avoid Strong Reference Cycles" section in "Transitioning to ARC Release Notes" (http://developer.apple.com/library/mac/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011226-CH1-SW4), this can be fixed using "__block", as the changes in the pull request.