Change http://ohmyz.sh link to use proper markdown syntax

Fix typo

A few spelling fixes, grammatical fixes (what's with all the ellipses?), some standardization of capitalization, and a few other tweaks.

As well, I changed all standalone code fragments into proper code blocks.

I also added a missing theme image!

Updating the install instructions as proposed in: https://github.com/robbyrussell/oh-my-zsh/issues/3516#issuecomment-76506726

Resolves #3786

Moving README to markdown... and taking first steps at reorganizing it. Also updated the LICENSE to reflect 2015.