Implement all-mighty `~>` operator

PR contains
- `~>` implementation. Alas, `Capture` approach does not. Most probably because of unshipped bug-fixes to Scala-2.10.3
- as a consequence other rules described in [issue#21](https://github.com/sirthias/parboiled2/issues/21)
- also: `test` and `EMPTY`

Thing are not trivial with `nTimes`. Also, I found some bugs by giving power of adding two and more elements to stack in a row. So, `nTimes` and bug-fixes are coming (in that or following PR).