Add sbt-dependency-graph plugin

I know `printDependencyClasspath` already shows classpath dependencies but the output is really hard to read. This plugins add following new tasks 


 * `dependency-graph`: Shows an ASCII graph of the project's dependencies on the sbt console
 * `dependency-graph-ml`: Generates a .graphml file with the project's dependencies to `target/dependencies-<config>.graphml`.
   Use e.g. [yEd](http://www.yworks.com/en/products_yed_about.html) to format the graph to your needs.
 * `dependency-dot`: Generates a .dot file with the project's dependencies to `target/dependencies-<config>.dot`.
    Use [graphviz](http://www.graphviz.org/) to render it to your preferred graphic format.
 * `dependency-tree`: Shows an ASCII tree representation of the project's dependencies
 * `what-depends-on <organization> <module> <revision>`: Find out what depends on an artifact. Shows a reverse dependency
   tree for the selected module.
 * `dependency-license-info`: show dependencies grouped by declared license
 * `ivy-report`: let's ivy generate the resolution report for you project. Use
   `show ivy-report` for the filename of the generated report