Rename init_shared_memory to prepare_memory #24

I don't like so much the @bot.init_shared_memory/add_shared_memory_initializer names: they're too long and a bit ugly. I think a better naming is the following one:

• @bot.init_shared_memory › @bot.prepare_memory
• self.add_shared_memory_initializer › self.add_memory_preparer

This is a breaking change, but the old syntax will continue to work until 1.0.