Initial idea for common NuGet logo

Thought it would be cool if all TestStack NuGets had same logo, when viewed on nuget.org and in VS package manager UI would look great if all TestStack projects had same "branding"

I went though a few design iterations, the one in this PR is optimized for the smaller view in nuget

You can visualize how the designs with names on aren't very readable, also as the names are usually seen next to the logo, I don't think the actual project name needs to be on the logo - for example look at the MS packages - they all use the same logo.

The .svg in the PR contains the vector design so it can be scaled in lossless way for other purposes (e.g. the docs landing page) You can use free Inkscape tool to view/edit.

The background color is currently metro purple but it's easily changeable, for example each TestStack project could have exact same logo but a different metro bg color, etc.

![tsmall](https://cloud.githubusercontent.com/assets/1472335/2677876/0c6a8e46-c159-11e3-9626-7c0d93e0bf08.png)

![tslarge](https://cloud.githubusercontent.com/assets/1472335/2677878/142e0978-c159-11e3-8659-bf02e477fdbe.png)