summary2 re-factoring
minor mix on group summary
Lock down Exec2 scalar answers

Fixed exposed bugs.  Still not testing array results.
is.na replaces Enum cols with T/F.
Uni ops return their input type plainly (if ary then ary, if dbl then
dbl)
Handle NA's in & | operators
Correct precedence in && ||
Merge branch 'master' of github.com:0xdata/h2o
Tone down a test
fix ASTCat bug
Merge branch 'master' of github.com:0xdata/h2o
Merge branch 'master' into tomk-ux