add client keepalive publish timing

Adding that clients send keepalive data every 20 seconds.

Changing example client config as the threshold was not technically possible at 10 seconds.

https://github.com/sensu/sensu/blob/master/lib/sensu/client.rb#L69
Merge pull request #77 from agent462/patch-2

add client keepalive publish timing
Added CentOS bootup instructions
Adjusted to create Start the RabbitMQ server section
Merge pull request #78 from VitaliyBSD/master

Added CentOS bootup instructions
Changed the command used for email handler

Sensu has to read the json data from stdin so the command in the doc
pre this commit will simply send out a blank e-mail with "sensu alert"
as the subject.
Revert "Changed the command used for email handler"

This reverts commit cb96f0295f47be61b26695c0c4f7e9d7272f2736.
Added a warning about default GNU mail