Add example join query
Associate a has_been_pushed variable with each selection conjunc
Use has_been_pushed variable in selection pushing
Function to compute fields accessed by an expression
Function to rebase an expression to a new column offset
Generalize logic for seleciton pushing
Reorder order of joins in join example
Fix up optimizer tests to reflect selection pushing
Rename rule to PushSelects
Test that combines joins with selection
Fix comment
Delete dead code
Merge pull request #141 from uwescience/andrew_push_select

Generalize selection pushing logic