CAS-914: encrypt ticket data in CAS ticket registries

Context:
https://issues.jasig.org/browse/CAS-914 

## Overview
This is a refactoring of the patch that @serac had proposed on the JIRA, which contains an Aspect that wraps itself around ticket registry calls. The pull is different in the following areas:

1. Secret key configuration is modeled after that of Clearpass, removing the need to create a separate key file. 
2. An option to enable/disable ticket encoding. Default is disabled. 

## Configuration

```xml
    <bean id="ticketEncodingAspect"
           class="org.jasig.cas.aspect.TicketEncodingAspect"
           factory-method="aspectOf"
           p:encoder-ref="ticketEncoder"
           p:enableTicketEncoding="${enable.ticket.encoding:false}"/>
 
     <bean id="ticketEncoder" class="org.jasig.cas.ticket.enc.SymmetricCipherEncoder"
           c:key-ref="ticketEncoderSecretKey" />
 
     <bean id="ticketEncoderSecretKey" class="org.jasig.cas.ticket.enc.SecretKeyFactoryBean" />
```