madlib.lm and glm utilities

I've added functionality to madlib.lm and madlib.glm to support the utility functions add1() and drop1(), and thereby step(). (They are also useful in themselves for assessing variable importance.) This involves:

- Modifying madlib.lm and glm to store terms and nobs components
- Modifying madlib.lm to compute and store the SSE
- Adding extractAIC and logLik methods for lm.madlib objects
- Adding an as.environment method for db.table and db.view objects; this basically creates a list of the individual columns in the table, and then turns the list into an environment