Changes to README and homepage

[digital-carver]

Hi,

I'm familiarizing myself with the project so I can get into qojulia/QuantumOptics.jl#407 with a clear idea, and came across some potential changes:

• Minor changes to README (typo fix, formatting, punctuation)
• Changes to index.html
  • Since the Github repo's About section says "Library for the numerical simulation of closed as well as open quantum systems", I removed "Open" from the main subtitle and also changed "various kinds of open quantum systems" in the description below that to "various kinds of closed and open quantum systems". Please let me know if this change is not warranted.
  • The phrasing of "by relying on different ways to store and work with operators" and "severely tested" sounded a little bit awkward, so I replaced those with more natural sounding wording.
  • Other than the above, just some very minor punctuation fixes.

I was planning on fixing the version number too, but the repo source says 1.0.1 while the website clearly says 1.2.1, so I'm assuming that's updated by a build script (that's not in the repo) somewhere along the way, so I'm leaving it as it is.