Optimizations and cleanups.

Optimization:

- write ARM assembly routine for biquads, using one load per pair of
  coefficients.

Correctness fix:

- clamp sound level estimate below at -50 dB because of integer truncation
  at estimateLevel() method.

Cosmetic changes:

- add headings in .cpp to make it easier to keep track of file structure.
- remove trailing whitespace
- rearrange some functions in logical order.