Polish installation and setup instructions

[stargaser]

Update the environment name and revise setup instructions.

• change environment name to astropy-env
• add instructions for installing git
• don't limit Mac/Linux users to bash shell for conda
• clarify when commands are typed in the terminal
• require 64-bit Miniconda
• be explicit where to start jupyter notebook

Fixes #165 , #170, #176, #54

[mwcraig]

Looks good -- I think changing the name of the environment will help reduce some confusion. Had a couple of very minor line comments.