Binary exercises

Hi, I took pull request #2577 by @EliotVU and did some polishing. The pull request contains the following new exercises:

- Binary operations 1: basic binary operations on single bits
- Binary operations 2: binary operations on bytes
- Binary operations 3: evaluating binary expressions
- Converting binary to decimal
- Converting decimal to binary

There are some new helper functions in utils/binary.js.

Sandcastle link: http://sandcastle.khanacademy.org/pull/11989/