Fix bs_skip_bits2

This instruction needs to fail when skipping
a negative amount.  e.g.

  << _:-8, … >> = Foo

should fail the match, not cause a runtime exception.
Improve ETuple.dump (debug class dump facility)
Improve re:compile output

Make it so that Erjang's re:compile can survive
being serialized (e.g. by the Elixir compiler)
Set target="1.7" in build file