added dj.set and dj.assert
use of dj.set to control internal behavior of datajoint
allowed nulls in all integers that are smaller than 64 bits
allowed prompt suppression in dj.Table/drop
fixed typos from last commit
fixed typos from previous commit
fixed typo from previous commit
Added type checking in dj.struct.tabulate. Only numerical scalars are allowed.
enabled oldValue = dj.set('param',value). Fixed issued #20.