CastingNode: fix bug in SQL generation

Apparently you can't puts an array of datasets as the value of of
a where clause.  It ends up returning a set of columns, one per dataset,
rather than a single, unified dataset.  Makes sense.
CHANGELOG: updates for 0.0.6
Gemspec: make ruby-graphviz a dependency
Tree/Node: each node now gets reference to tree
Define/Recall/Tree: pass types between Define and Recall using Tree
Node#sql: add this function to return SQL executed by nodes
Graph: now includes row counts at each step!
CHANGELOG: update
Dottable: include patient count with row count
Dottable: show dashed lines if no results are found
Merge branch 'master' into develop