Closes #5558

https://github.com/robbyrussell/oh-my-zsh/wiki/Themes#agnoster

Change OS X to macOS in the read me file in line with apples rebranding
of the operating system.

From `raw.github.com' to `raw.githubusercontent.com'

Fixes #4933

Also, fix a couple of typos / style issues.

The tilde character is not expanded when inside quotes. See https://github.com/robbyrussell/oh-my-zsh/issues/4668#issuecomment-162213379

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.