Documentation improvements

[mtfishman]

Here is an issue to keep track of things to do related to documentation.

• Add instructions on installing MKL with the MKL package.
• Add instructions for turning on/off debug checking (as explained here: Debug checks #323).
• Add instructions on developing ITensors.jl, for example using the dev ITensors command and using Revise.
• Add instructions on installing OhMyREPL for syntax highlighting in the REPL, and mention using the command enable_autocomplete_brackets(false) to turn off automatic matching of quotes/brackets (which can be very annoying when making primed indices).
• Add explanations about precompiling, slow startup times, etc. In addition, add some instructions on using PackageCompiler with ITensors.jl to speed up startup times.
• Add docs for converting ITensors to Arrays.
• Add keyword argument options for dmrg.
• Improve IndexSet, ITensor, MPS/MPO docs.
• Add docs for QNs.
• Add docs for AutoMPO, and other dmrg-related functionality.
• Add docs for Sweeps.
• Add more doctests.
• Host the docs on Github and integrate documentation building and doctests with Travis.
• Fix formatting issues.