feat(NodePath.prune): deal with an IfStatement not having its consequent after a prune.

If an IfStatement has it's consequent pruned and there is no alternate leave the test as
a ExpressionStatement. If the alternate still exists negate the test and move the alternate
to the consequent position.
feat(Node.prune): do not add double negation to if test expression.

Also addressed code review concerns.
Merge pull request #71 from briandipalma/master

NodePath#prune: deal with an IfStatement not having a consequent.