JaaS support added for OSGi JVM Agent (fixed #139 and #140)

Initial idea and implementation comes from Piotr Kania <pkania@gmail.com>, see also pull request #140. The final implementation has been streamlined and adapted a bit.

* Instead of 'role' a 'authMethod' config var has been introduced (with possible values 'basic' and 'jaas')
* Roles are not checked explicitely but only a login is performed. Authorization has been checked by the server runtime (e.g. Karaf) and will fail then. This makes the implementation simpler and less dependent on Karaf.
* Refactored into smaller classes
* Added support for JVM Client
* Use jmockit for mocking JAAS

New Configuration vars:

* org.jolokia.realm  -- Realm and JAAS Domain
* org.jolokia.authMethod -- "basic" or "jaas"

If a JAAS login was sucessful, then the JAAS subject is copied into the request attribute "org.jolokia.jaasSubject" which is checked and used during performing JMX operations. If using a WAR agent you probably need to add an own Servlet filter for performing login and setting of this request attribute.