add aggregate operation

[preyunk]

Add support for aggregate operations on queryset,
user can pass multiple aggregate operations, also all operations are stopped after aggregate.

Added test cases

[vmwclabot]

@preyunk, you must sign every commit in this pull request acknowledging our Developer Certificate of Origin before your changes are merged. This can be done by adding Signed-off-by: John Doe <[email protected]> to the last line of each Git commit message. The e-mail address used to sign must match the e-mail address of the Git author. Click here to view the Developer Certificate of Origin agreement.

• Commit a0b96ea must be signed by [email protected]

[vmwclabot]

@preyunk, you must sign every commit in this pull request acknowledging our Developer Certificate of Origin before your changes are merged. This can be done by adding Signed-off-by: John Doe <[email protected]> to the last line of each Git commit message. The e-mail address used to sign must match the e-mail address of the Git author. Click here to view the Developer Certificate of Origin agreement.

• Commit a0b96ea must be signed by [email protected]

[piyusql]

Looks good