Support for mailbox bounds (for issue 146)

Hello. Perhaps this may help with issue #146.

This commit introduces the possibility of limiting the size of a `Celluloid::Mailbox` to given number of messages. This limit can be set by executing something like:

```ruby
actor.mailbox.limit = 5
```

A message sent to a full mailbox will raise a `Celluloid::MailboxFull` exception.