Improve db.objects

Made it easier to delete multiple tables
Added as.db.data.frame for db.data.frame

to allow copy of the data
Fixed the problem of as.db.data.frame with file

No need to use file's full path anymore
Added example for array fitting to doc
Finished the code for sample
Fixed a small bug:

delete cannot rm the object in R after the table is dropped
Finished coding of generic CV

Also fixed a bug: sampled data is a db.data.frame, must converted to
db.Rquery before expanding to dummy variables
Added generic bagging with average predict

vote predict will be added soon
Added vote support for generic.bagging

Also create temp tables in generic.cv to save time
Added deletion of temp tables in generic.cv
Added doc skeleton to pass check
Made the skeleton doc for sample correct
Finished doc for sample
Added doc for generic.bagging
Added doc for predict.bagging.model
Added doc for generic.cv
Improve doc format a little bit
Fixed lots of bugs in generic.cv and generic.bagging

* Tests for multiple parameter cv is still needed

* More complicated tests for bagging would be helpful

* 'c' is confusing to use because it is already overridden. Need a
  better way to call 'c'