approve() race condition mitigation

Add destroy function, allowing send funds to recepient.

Revert to usig npm as preferred installation method

Add external audit report

Create and test PausableToken Contract

Await event handling result in Bounty test

Make SafeMath a library

Update README with new slack url

Added totalPayments uint on PullPayments contract

Add MintFinished event to MintableToken

Remove excessive condition from SafeMath.safeAdd()

There is no situation when `c>=a` will be `true` while `c>=b` will be `false`.

fix for short address attack

Create and test MintableToken contract

as suggested by izqui in case a function calls transfer under the hood

Change killable to destructible and kill to destroy

as suggested by /u/izqui9 here https://www.reddit.com/r/ethereum/comments/63s917/worrysome_bug_exploit_with_erc20_token/dfwmhc3/
Attack description: 
https://blog.golemproject.net/how-to-find-10m-by-just-reading-blockchain-6ae9d39fcd95

Follow security audit recommendations