Adding RuleBaseReader for rules defined in JSON format.

Motivation:
The ProxyServlet class provides the abstract loadRuleBase method in
which concrete implmentation can define the proxy rules. Currently
subclasses can use the Java DSL, or whatever way they chose to read the
rules and make them available. This pull request provides support for
defining rules in JSON format.

Modifications:
* Added a dependency to com.fasterxml.jackson.core:jackson-databind.
* Added RuleBaseReader.