Full support of YAQL evaluation

Mistral evaluation of YAQL expression is mostly string substitutions today. There are other complex YAQL expressions that should be supported (arithmetics, containers, boolean, etc.). There's a bug in the regex @ https://github.com/stackforge/mistral/blob/master/mistral/expressions.py#L82 that some YAQL expression that starts with $ gets evaluated (i.e. expr.evaluate('{$.i in $.list}', {'i': 1, 'list': [1, 2, 3]}) works but not expr.evaluate('{1 in $.list}', {'list': [1, 2, 3]})). So what level of YAQL support mistral has is ambiguous.