pom.xml: lower the required version of Maven to 2.2.0

No features in Mupd8 seem to depend on Maven 3.3.0.  This fix lower the required/minimum version.
Merge branch 'sam-dev', remote-tracking branch 'origin/sam-dev' into sam-dev
source-reader: modify source reader behavior to match test requirement

This patch changes the source reader at interface level and concrete implementation level to accommodate the requirement specified in the JSONSOurceTest (See commit zheguang/mupd8@c665ef2c2ae8d69a485310b90dd4b1dae04431b5)
source-reader: refactor and code-change for Scala idioms

This patch refactors the JSONSource to increase readability. Also, the usage of null is changed to Option to favor Scala idioms.