Resolves issue #11: Now supports server version 3.2. Changes made in this PR:

[dongxinEric]

• Added support of the command 'find'. Note it only supports following options:
  • filter
  • sort
  • projection
  • hint (will be parsed out but not actually used yet.)
  • skip
  • limit
  • batchSize
  • maxTimeMS (will be parsed out but not actually used yet.)
• Added support of the command 'getMore' which will be used along with the new 'find' command;
• Added support of the command 'killCursors' which will again be used along with the combination of 'find' and 'getMore'
• Added support of the command 'explain' which will be used after 3.2 instead of the old OP_QUERY way

With these newly added commands, DocLayer server now broadcasts server version 3.2 to all incoming connections.

Will added tests in later commit(s).

Manually tested Swift driver (MongoKitten v5.1.11), Go driver (Mongo Go v3.0 and v3.2) and PyMongo-3.6.1. All worked fine. Note that all our existing pytests are based on PyMongo-3.6.1 which will automatically switch to use all the new commands if the server says it's on version 3.2, and since all our tests can still run fine, that's a good indicator that PyMongo will just work fine with the changes.

@apkar is still on vacation, but he will review this once he's back.

[dongxinEric]

Forgot to say this will resolve #11