Enhance ezbake to support extra uberjars for JRuby 1.7 vs 9k

In looking into the work to support the configuration of JRuby 1.7 vs. 9k uberjars for SERVER-1630, we realized that we're going to need some changes made to ezbake. The ezbake work will likely consist of:

1) Provide an optional mechanism in a project's ezbake profile config to specify:

• A set of Maven coordinates (or maybe GitHub repo + treeish) for a set of "additional jar dependencies" to retrieve.
• A default set of files to add to the Java classpath for the start, foreground, ruby, irb, and gem scripts.

2) Provide a way for users to configure a new environment variable in the sysconfig/default script which, when set, is appended to the Java classpath in place of any default configured in the project ezbake configuration.

3) For any "additional jar dependencies" set in the ezbake configuration, modify ezbake to:

• Retrieve project source or Maven artifact (immediate jar + pom.xml/project.clj dependencies).
• Build an uberjar (immediate jar + any upstream dependencies) of each retrieved project.
• Package up each additional uberjar alongside the main project's uberjar.

4) Modify the ezbake.manifest to include information for any additional uberjars which might be built into the package.