add other reaction regulators to BEL model

[tythomson]

Other (allosteric) regulators of reaction are not currently included in the BEL mode. For example, in R-HSA-70589.1 ADP is a negative regulator of the reaction and ATP is a positive regulator of the reaction. These might be represented by the following relationships although I couldn't find much in the reactome documentation about this so I could be wrong: PositiveRegulation and NegativeRegulation (http://wiki.reactome.org/index.php/Glossary_Data_Model). In BEL they could be represented with causal 'increases' and 'decreases' edges.