v0.2.2

* Performance Upgrades
  * Added BetaNode indexing
  * Abstracted out JoinNode to extend BetaNode (Prevents the checking of constraints if there are not any constraints to check)
  * Refactored BetaNetwork
  * Created a new Memory Class to encapusulate left and right memory for BetaNodes
* Added new `exists` logic operator to check for existence of fact (opposite of `not`)
Merge pull request #91 from doug-martin/master

v0.2.2