ENH: add get_average model
CHG: quantiles in average model are stored better
BUG: afew fixes to the quantiles related methods
merge qp_plot
CHG: rewrite of data_plot
MIN: small fix to data_plot
CHG: change range of params generated by gen_rand_params
Merge branch 'master' of https://github.com/hddm-devs/hddm