Secret Document Objectives

Adds new steal objective ![docsgen](https://cloud.githubusercontent.com/assets/3828067/2874072/00837f60-d3d4-11e3-860b-f72a7ba5c249.png) "Secret Documents"
-A set of secret documents spawn in the vault's filing cabinet, but even syndicate documents qualify as valid steals

Adds the exchange objective for traitors
-If there are at least 6 traitors, two of them will be selected to make an exchange of secret documents
-There is a low (20%) chance that one of them will have an additional backstab objective, where they will require both sets of documents to win
-Sending syndicate documents to centcom will yield a very large point reward