make Reaper accessible in Bot

[maxime915]

This commit is a refactoring of the types bot, Bot, account, lurker, scanner and reaper (with mockReaper). The goal is to make reaper available outside the package (as #59 requests). Detailed modifications:

• Renaming "reaper" to "Reaper" to export the interface,
• Exporting methods reap -> Reap, sow -> Sow, get_sow -> GetSow, rateBlock -> RateBlock,
• Adding RateBlock to the new Reaper interface. To avoid breaking compatibility only private interface/type were modified, one interface was exported with a new method. No new tests were added : the exported methods are already tested and no new functionalities were added.

[CLAassistant]

All committers have signed the CLA.