Sketch versioning scheme

As we discussed in the mailing lists: http://markmail.org/message/c2mcomocs7x2b4bc

in order to move forward with our first release a detail we have to take a look is how the versioning scheme of our two artifacts.

The current proposal to validate is:

• both get released separately
• api is always versioning without patch number (i.e., only x.y)
• simple impl can release patch versions (x.y.z), but then it's clear the
  api version it is implementing