Exec2: env.remove uses unchecked pop
Merge branch 'master' of github.com:0xdata/h2o
Merge branch 'master' of github.com:0xdata/h2o
New changes to exec2: Inf, min.na.rm, max.na.rm

Add Inf as global constant
add operators min.na.rm and max.na.rm, which act as R's min(..., na.rm=T)
Add sum.na.rm to exec2.
Merge branch 'master' of github.com:0xdata/h2o