JCR-2113: add access control mgt support for jcr remoting

This patch provides acm support for the jcr remoting client. in this patch, the implementation of the following has been realized:

1. implementatiion of javax.jcr.security.AccessControlManager api for jcr2spi jackrabbit client that allows for the creation, retrieval and removal of access control policies.
2. jackrabbit.api.security.JackrabbitAccessControlList as the default acl policy implementation for the client.
3. jackrabbit.api.security.JackrabbitAccessControlEntry: the entries that goes in the acl.
4. extensible access control manager implementation.  creation of an access control manager is done via an AccessControlProvider, which is dynamically loaded and instantiated from a configured properties file. See  SessionImpl#getAccessControlManager() for details.

/cc @anchela @tripodsan @dbu @alexparvulescu @mduerig
to me it still looks a bit odd to completely drop the 'createItem' method and replace it with 'importItem'. this means that you are always using the extra conversion to xml even for those cases where it would not be required... see comments in the corresponding section.