Routing alerts to appropriate mailing lists and hipchat rooms depending on the client executing the check

The change:
1) The mail_to attribute in the "mailer.rb" handler script can now be obtained from the client attributes as well where the check was executed and the event was sent from. It allows us to have one check config which different project environment clients can subscribe to and the mail alerts could be directed to the appropriate mailing list based on what environment was the client part of. Earlier, one check config could only be associated with a single list of mailing lists and all clients executing the check would fire alerts to each of the email ids specified in the mail config. This forced each project to have their own copy of check and handler config defined with their own specific attributes to the mailing script. So, in spite of the check being the same such as some system level check like cpu usage/disk usage, across multiple projects, each project had to define a copy of the same check config and handler config with the attributes tweaked. Instead, if the mail parameters could be specified in the client config itself, the same check and the same handler can fire alerts to project specific mail ids.

2) The same implementation as above for routing alerts to project specific hipchat rooms.

Change required on the client attributes to use the above functionality:

Addition of the following params in the client json:
"hipchat_room": "<room name",
"mail_to": "<comma separated emails"

Note: The change is backward compatible and if the above params are not available in the client attributes, these are picked from the handler config (mailer.json and hipchat.json) which serve as the default configs.