Merge pull request #20 from sacjaya/Siddhi-3.0.0

removing validation classes
refining exception hierarchy
adding support for functions
adding support for aggregated attributes
Merge pull request #21 from sacjaya/Siddhi-3.0.0

Changes based on code review
Extension support
Length window implementation
suggested improvements at code review
Merge branch 'Siddhi-3.0.0' of https://github.com/wso2-dev/siddhi into Siddhi-3.0.0
Merge pull request #22 from tishan89/Siddhi-3.0.0

Siddhi 3.0.0 - suggested improvements at code review
code quality improvements
Merge branch 'Siddhi-3.0.0' of https://github.com/wso2-dev/siddhi into Siddhi-3.0.0
Merge pull request #24 from tishan89/Siddhi-3.0.0

Siddhi 3.0.0 code quality improvements
Merge pull request #23 from sacjaya/Siddhi-3.0.0

test cases for extensions
modified returnEvent method of eventPool
Merge branch 'Siddhi-3.0.0' of https://github.com/wso2-dev/siddhi into Siddhi-3.0.0
code quality improvement