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.
Javadoc fix here.