Fix spelling of its vs it's in source and documentation.

To disabiguate the two homophones, the possessive form
is spelled "its", without an appostrophe. The contraction
"it's" retains the appostrophe. A good way to check which
to write is to try replacing "it's" with the expansion "it is".
If that doesn't make sense, it's the possessive, and should
instead be "its".
Merge pull request #4 from rillian/master

Spelling fix in documentation and code comments.