optimize away logical negation if possible

the AST is checked for logical negations of comparison operations
if appropriate, the negation is converted into the reverse comparsion operation, e.g.

    NOT (a == b) => (a != b)
    NOT (a != b) => (a == b)
    NOT (a > b)  => (a <= b)
    NOT (a >= b) => (a < b)
    NOT (a < b)  => (a >= b)
    NOT (a <= b) => (a > b)
Merge branch 'aql2' of https://github.com/triAGENS/ArangoDB into aql2