Feature: Adding jump based repeat pattern to GripQL [WIP]

[kellrott]

This PR adds the ability to create dynamic length graph traversals using GripQL. This is done via a 'mark' and 'jump' based interface, similar to using a 'goto' statement, but for the stream processor.

While more primitive than the repeat mechanisms seen in Gremlin, this allows for a much more simple query compilation and implementation.

The PR adds 4 new operations:

• mark(name) : Mark a segment in the stream processor, with a name, that can receive jumps
• jump(dest, condition, emit) : If a condition is true, send traveler to mark. If emit is true, also send a copy down the processing chain. If condition is None, always do jump.
• set(field, value) : Set values within the travelers memory. Can be used to store cycle counts
• increment(field, value) : Increment field, ie data[field] = data[field] + value. Can be used to increment counter every cycle

Example queries:

q = G.query().V("Character:1").set("count", 0).as_("start").mark("a").out().increment("$start.count")
q = q.has(gripql.lt("$start.count", 2))
q = q.jump("a", None, True)