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".
two more implementation pointers
Merge pull request #4 from rillian/master

Spelling fix in documentation and code comments.
Merge pull request #7 from dwaite/master

numerous changes to formatting and sentence flow
mention DER format, need to improve line id / shared secret definition
Fixes a typo
Simplify explanation of hashnames
Mention Sybil attack first, then show the code

Makes more sense when reading the whole spec.
Merge pull request #8 from aflatter/master

Some small changes after seeing @dwaite's huge commit
better describe separation of stream and state