Whitepaper roughdraft in tex; gitignore update

Whitepaper added to the doc/ folder, is currently incomplete and in
rough draft status. It's mostly complete and accurate up to the 'Proof
of Access' Section.

gitignore updated to ignore all .pdf, .log, .aux, and .toc files,
because compiled latex and logs should not appear in the repo.
Merge branch 'master' of github.com:Siacoin/Sia
Merge pull request #36 from DavidVorick/master

Add whitepaper to /doc