TACHYON-120 Fail to cast String to other objects for JsonObject

Currently JsonObject.get just directly cast parameter Object to the generic T type. 
This could generate class cast exception such as the one from String to DependencyType.

Modify the code to use Jackson JSON formatter instead by passing the Class<T> type in the get method to avoid Java type erasure to get the actual parameterized type.

Extra fix:
Fix Javadoc parameter name
Remove unneeded Javadoc exception throwing
 ele.<List<Integer>> get() worked before. 

Could it use get(name, List<Integer>.getClass()) instead?