Merge branch 'stable'

Update erlang.mk

Minor doc improvements

Merge pull request rabbitmq#32 from rabbitmq/rabbitmq-server-20

Unsigned field types to support AMQP-0-9-1

Merge pull request rabbitmq#30 from rabbitmq/rabbitmq-server-413

logs warning when queues_to_recover > available file handles

Ranch already handles option sanity checking, so this can go

Merge pull request rabbitmq#13 from rabbitmq/rabbitmq-server-419

More sensible error messages for connection-level exceptions on running connections