Fix some memory leaks and refactor the memory management code.

# Fixing Memory Leaks
1. Free the utf8Text according to the comment in Tesseract that “The recognized text ... must be freed with the delete [] operator.”.
2. Add freeing of _pixels and _tesseract itself.

# Refactor Memory Freeing Code
1. Move all freeing memory jobs of Tesseract to its dealloc and make - (void)clear deprecated.