Solution for issue #414. Partial solution for #401

The backtracking for the or operator is now solved as follows:
- a muOr operator added to muRascal.
- a JMPINDEXED instruction added to RVM.

The code generated for || maintains the current alternative and jumps to
it on backtracking. && and || now completely implement short circuit
semantics. The remaining operators (see #401) still need to be adapted.

Note: SendMoreMoney now loops. Not yet analyzed and disabled as test
Merge branch 'master' of https://github.com/cwi-swat/rascal.git