Documentation fix-up: "its" vs. "it's"

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.
Merge pull request #91 from smurfix/de_apostroph_ify

Documentation fix-up: "its" vs. "it's"