Make it feature complete with the vault API

[gerrrg]

We're missing few misc functions for the vault that would be nice to have just for the sake of being feature complete. (getAuthorizer, hasApprovedRelayer, etc.)

[gerrrg]

https://dev.balancer.fi/references/contracts/apis/the-vault

[gerrrg]

Easy ones:

getAuthorizer()
hasApprovedRelayer()
setRelayerApproval()
getPool()
getPoolTokenInfo()
getProtocolFeesCollector()
setPaused()

A bit more time consuming: exitPool()

For exitPool, use joinPool as an example