mem-helper: move align-mem functionality to class

There was some memory alignment logic inside the Mixer cpp file. To break down
the code and separate things into smaller modules, the aligned memory stuff
was crammed into a new class called MemoryHelper.

The MemoryHelper can be reused for any other aligned memory that may be needed
by another component.
mem-helper.cpp: add license information
Merge pull request #617 from psyomn/local-master

Move align-mem functions to Class