Add support  for rhc app make-ha

Currently when creating an ha application you have to do it via curl, for exemple

    $ rhc create-app testha ruby-1.9 -s
    Your application ‘testha’ is now available.

    $ curl -k -X POST https://broker.example.com/broker/rest/domains/test/applications/testha/events –user demo:password –data-urlencode event=make-ha

This pull request add support for `make-ha` in rhc

    $ rhc app make-ha -a testha
Looks good, but add a check to make sure the method is supported so we get a nicer error message:
```
      def make_ha
        if supports? "MAKE_HA"
          rest_method 'MAKE_HA', :event => "make-ha"
        else
          raise RHC::HighAvailabilityNotSupportedException.new
        end
      end
```

And define HighAvailabilityNotSupportedException in lib/rhc/exceptions.rb like this:
```
  class HighAvailabilityNotSupportedException < Exception
    def initialize(message="The server does not support high availability")
      super message, 135
    end
  end
```