Add ModEM logos to README and Documentation

[MiCurry]

Figured I'd add these logos to make things look more official! Please let me know what you think!

I added a logo to the readme, you can see it here: https://github.com/MiCurry/ModEM/tree/docs/logos

Here is how it looks on the documentation, I think it looks pretty good:

[akelbert]

This all looks good to merge! But please edit the first few paragraphs in the documentation, they sounds rather strange, especially the part that starts with "While a primary design goal of the system..." - please replace the first two paragraphs with the following combined text.

ModEM was designed as a flexible electromagnetic modeling and inversion system, written in Fortran 95. A primary design goal of the system was to allow simplified extension with regard to data types, modeling codes, parameterization and regularization, and inversion search algorithms (see Egbert and Kelbert, 2012; Kelbert et al. 2014). Although the code can be (and has been) extended for inversion of more general types of EM data (e.g., controlled source, DC; see Meqbel and Ritter, 2015), here we describe the stable, core system developed for 2D and 3D magnetotelluric (MT) problems. The stable MT program has a command-line interface, which controls available program options, and specifies required and optional input and output files.