Entity-Relationship Diagrams for Mdm and Mdm::Module

[#54771390]
ER Clusters

[#54771390]

Allow the generation of clusters from a given set of ActiveRecord::Base
subclasses by generalizing the code for the Mdm::Module diagram.  Also
add erd:maximal_clusters that gives a list of the maximal clusters
reachable from each individual class as a guide for creating more manaul
clusters like in rake erd:mdm:module.
Include ERD in YARD docs

[#54771390]
Fix RailsERD association warnings

[#54771390]
Fix svg inclusion

[#54771390]

.svg file output from graphviz contains the XML declaration and various
comments before the root <svg> tag, but the the declaration and
comments are visible in the generated YARD docs, so remove them from the
file that is included by YARD.
Add ERD so it shows up on rubydoc.info

[#54771390]
Use png asset instead of svg for ERD

[#54771390]

rubydoc.info would not display the svg.inline correctly: it sanitized
the code and showed the svg statements instead of embedding the svg tags
inline, so switch to using a png and then use the --asset option to mark
the image available in markdown.
Mdm::Module SQL translation guide

[#54771390]
Merge pull request #34 from rapid7/feature/erd

Verification steps all passing; translations look like they will be most helpful. [Delivers #54771390]