Refactored lm and glm
Modified madlib.* doc to emphasize the parallel computation
Let as.db.data.frame print message by default
Update pkg.Rd to add plot random sample
Update README.md
Update README.md
All data types are converted to lower cases

'ARRAY' should be 'array' in .is.array
Suppress warning in preview
Update README.md
'c' should only work on a single column
Changed 'c' to 'cagg'

'c' has its own usage to combine multiple db.obj objects.

'cagg' will reduce the confusion