Enable buidler evm to support exact timestamp testing

[tranvictor]

Changes:

1. Allow evm_mine to accept a timestamp param to mine an empty block at the exact timestamp.
2. Add new evm_setNextBlockTimestamp to register a timestamp for the next block to be mined at.
3. Add the doc for evm_setNextBlockTimestamp.
4. Add relevant tests for those changes.

Noted behaviors:
Regarding evm_setNextBlockTimestamp, it requires one and only one timestamp param. It set the timestamp as a field named _nextBlockTimestamp of the node, this field is taken and restored as per snapshot operations, just like blockTimeOffsetInSeconds. Once the _nextBlockTimestamp is set, the next block will be mined with the timestamp set to its value, EXCEPT if the next block is mined with evm_mine(timestamp. Whenever a new block is mined, _nextBlockTimestamp is reset to 0 as its null state.

Usecase:
For smart contracts that have block timestamp logic, it is necessary to test its edge cases where a transaction is mined at an exact timestamp.

[alcuadrado]

Really good work, Victor! Left a few minor comments.