Add visual to Frequency Spectrum goodie

Re #300, adds visual (inline SVG) for electromagnetic and/or audible frequency ranges.
![image](https://cloud.githubusercontent.com/assets/726198/3488558/cb77b122-04ef-11e4-9899-26da233c6315.png)

It looks like you tightened up the number matching part from my initial suggestion which was a very good idea. I did some more extensive testing than I was able to do against my initial version and it looks like this does exactly what it says on the tin.

Could you add an extra test or two to show it not triggering on, say, "kilo hertz" and "jiggahz"?

Could you add some tests for