Using "parallel" gem to parse files in parallel.

To make this performance-boost work, some restructuring was required:

- Parser now includes DocParser, returning the doc-blocks
  in fully parsed form.

- Aggregator no more includes Parser.  Instead of parse() method
  it now has aggregate(), which expects output of Parser as input.
  Because of this some changes to unit tests were also needed.

- App performs the parallel parsing of source code and then
  feeds the results of parsing sequencially to Aggregator.

- Timing now reports three measures: parsing, aggregating, generating.
  But because of the parallel-phase, the report of parsing isn't
  that accurate as it also includes the generation of code files.