Introduce Bunny::NotAllowedError
Make sure connection.tune.channel_max does not overflow

It is a signed 16 bit integer in the spec.

Fixes #174.

Conflicts:
	lib/bunny/session.rb
Add a test that demonstrates connection with overriden channel_max

Conflicts:
	lib/bunny/session.rb
Shutdown old heartbeat sender when initializing a new one

References #172.
Change log update
Change log update