Update include images that still say Mastercoin or MSC.

[msgilligan]

There are three external images in the current draft v0.7 of the spec. All three contain the word "Mastercoin". At the very least new PNGs can be created.

1. Reward formula

2. Architectural Layers

3. Class B Obfuscation Example

[msgilligan]

It may be possible to replace images with markdown syntax:

1. For the Reward Formula, we might be able to use Asciidoctor's math expressions support.
2. For the Architectural Layers, we might be able to use Asciidoctor Diagram
3. There should be a text/markup-based alternative for the Obfuscation Example, as well.

[msgilligan]

PR #346 was an attempt to replace the Reward Formula PNG with markdown, but for some reason the Github software isn't processing it correctly.

[Oscarwong79]

#345