Handler order reversed for handlers with the same priority.

Previously, when handlers have the same priority, the handler
added last would have a higher priority.  This has been swapped so
that they are FIFO ordered.
Reduced the number of require statements across the rake task files.
Removed a referenced plugin that does not exist in this branch.
Refactored Seahorse::Client::HandlerList to make entries more visible.
Fixed an issue with the content length plugin priority not being set correctly.
Fixed a bug with Glacier compute checksums on empty bodies.
Running `rake -T` with ENV['ALL'] set will document a large number of other tasks.

For example

    $ ALL=1 rake -T

This will display description for all of the service specific tasks.
Added a rake task that will dump the list of handlers applied to a service.

You can list the default service handlers like so:

    $ rake handlers:s3

To dump the list of default handlers for a specific operation:

    $ rake handlers:s3:put_object
Merge branch 'master' of https://github.com/aws/aws-sdk-core-ruby