sort iterator basic function already done!!!
print and the projectionIterator for the variable class is ok!!!
filter and scan can be supported by variable schema and blockstreambase
begin to write the hashtable2 supported by variable version
project physical iterator
a funciton point to support the project!!!
remove reduntant item
Merge branch 'Client & Server'

Conflicts:
	.gitignore
try to merge the data type codes contributed by dsc
data type promotion
add the doc which contains some images and documents.
change the image to jpg.
change the image to jpg.
a synchronizing point for zhuhe and kai
+ for arithmetic expression, < for compare expression,
CASE..WHEN..THEN..ELSE for composite function.
test utility
add setValue methods to ExpressionItem to facilite the intialization.
can do a a+b-1, a-b+1 at the physical level.
project and sort.
project logical operator can be done.