Extract specific phosphosite binding information from Phospho.ELM

[jmuhlich]

Phospho.ELM is full of detailed information like "EGFR phospho-Y1092 binds the SH2 domain of GRB2". This could help us build a better picture of when certain binding events should be exclusive/competitive as well as properly handle multiple binding (e.g. GRB2 also binds EGFR phospho-1172).