Add Thor task to download, unpack, and set up eXist

This commit unpacks the eXist .war file into a new directory. Because
the webapp is exploded and the filepaths slightly differ, the new
task checks for older path references and updates them. It also
changes eXist's default ports to reflect Jetty's.

The new task is intended to configure eXist, removing the need for
cerberus_core's Exist Generator.

To run the task, use "thor exist_jetty:init".
Edit eXist's default security permissions

When Jetty is running, the task will try to change eXist's default
admin password (an empty string) to "dsgT@pas". It will also
change the default database permissions from "rwxrwxrwx" to
"rwxrwxr--". This allows eXist to work fully out-of-the-box, since
eXist does not allow modifications while the admin password is
an empty string.
Merge pull request #8 from NEU-Libraries/provision-exist

Provision exist