implement block timestamp helper functions

[topocount]

• add blockTime helper to get a block's timestamp or estimated mining date
• implement a boolean helper that allows for different language depending on whether the block in question has been mined or not.

[coveralls]

Coverage increased (+0.3%) to 90.037% when pulling 2979c12953be74bc04f491b89f4188512aacb11a on topocount:feat/blockTime into 4aef5904dbe75b7bbd6186f008ee7774a229892f on aragon:master.

[topocount]

Could you also remind me where you were intending to use these specific strings? Having strings that may change based on time means that we will need to refresh / invalidate caches in places that save rendered radspec strings (e.g. Voting descriptions).

Thanks for the thorough review Brett! We see these being helpful for use with rewards radspec strings in terms of providing helpful details for votes on rewards, mostly. We also more generally see these helpers allowing for more apps to be built around logic that relies on future block numbers, as opposed to timestamps, ie more token distribution use-cases where block timestamps aren't as useful.