Group the skippable elements by packets of 10

Group skippable elements instead of putting them all in one single
sentence, so that the audio buffers won't grow too much.

Also:
- Fixed regex group capture that made skippable elements to range only
  from 0 to 10.
- Added tests
- In the tests: changed some paths that were not synchronized with the
  current version of text-to-ssml
Merge pull request #18 from daisy/memfriendly

Close #18