modifications to install instructions

[gregcaporaso]

Detailed changes:

• dropped anaconda installlation instructions in favor of pointing readers to their website (since your instructions could easily become out of date)
• dropped enumerated dependencies - interested users can easily see that info from the install command. The list would quickly become out-dated, and honestly most users don't care.
• changed created environment name to st2 to be more specific (I have tons of Python 3 environments, as will many users, so py3 would be confusing).
• updated the install steps so the user doesn't have to clone the repo to install (most users won't necessarily want to have the source code locally)

[lkursell]

Agreed that looks much cleaner

[gregcaporaso]

Thanks!