* Update for v 0.19

* Add rnotes from github releases page

* Add section on graph inspection tools, add link to HE transformer, and fix link to ngraph bridge for Tensorflow

* Fix broken links

* Remove MANYLINUX from list of default flags

* Revise some for new script loc

* Add stub for ngc_util python script

* Might as well link to open source file

* original file from Vasanth Tovinkere

* updated to handle more attributes.

* Update new script loc and details

* Revise intro

* Clarify large graphs generated from script best read on gephi or cytoscape only

* Edit oddly-worded sentence for clarity

* Delete ngraph_json_to_graphml.py

* Delete ngc_util.py

* Add nGraph logo, update color scheme, disallow preprocessing of frontends' markdown UI

* Finalize doc changes for release v0.19

* PR feedback on nGraph-friendly tools