Flexibility to have multiple hipchat/mailer configs

Please refer: https://github.com/sensu/sensu-plugin/issues/66

Currently, the handler assumes that there will be a json file which will have "hipchat" as the top-level key in the json. Due to the same, we can only have one hipchat configuration for using with hipchat.rb.

The above changes the behavior by taking in an optional config json top level key name while invoking the handler (Instead of "hipchat.rb" you invoke "hipchat.rb --jsonConfig <config top level key>" as the handler command).

In this way, we can have multiple handlers defined with different hipchat configurations such as different hipchat rooms or different from fields.

Note: As it is an optional parameter, it will just work fine with the existing hipchat.json method (Backward compatible :) )