Nested object comprehensions

Refactoring makeReturn to accept as an argument the type of return
(array or object key-value) required.
Fixes gkz/LiveScript#560
Object comprehension supports let

Fixes gkz/LiveScript#519
Merge branch 'issue294' into object-comprehension-fixes
Merge branch 'issue519' into object-comprehension-fixes
Merge branch 'object-comprehension-fixes'
Merge pull request #572 from rhendric/master

Object comprehension fixes