Split into a core package that depends only on shiro-core and a servlet package that depends on shiro-web

buji-pac4j depends on the Shiro Web module, which is largely Servlet-specific at the moment. [In Shiro 2.0, shiro-web may be renamed to shiro-servlet to better reflect what it does](https://cwiki.apache.org/confluence/display/SHIRO/Version+2+Brainstorming)

There are some things I'd like to use in buji-pac4j such as the ClientRealm that depend only on shiro-core and not on shiro-web. I'd like to use this library in my Play Framework where shiro-web really is not appropriate because it is not a servlet environment.

This refactoring breaks the project into two modules. One core module which can be used in non-servlet environments and another which depends on that module and is meant for being used with j2ee servlets.