Add 0xcert vesting contract

[lknix]

I wanted to open this one when #5 gets in, but since I already have the code, here it is. Unfortunately I had to branch off #5 to keep it clean and have access to some helper functions.

This contract sets up vesting periods for founders, team members and advisors.

To discuss:

• vesting periods are measured in approximate months which should probably be changed to days or weeks;
• current vesting start timestamp is ICO's start ts. Even though tokens are still locked until the end of the ICO period (when token transfers become available for everyone).