Multiple measurement support

Allows for either "measurement" or "measurements" to be included in the payload.  This is currently based off of master for testability purposes. Probably should rebase this off of the gemify branch, right?

* alert tests account for both "measurement" and "measurements"
* customer_io screenshot not included since functionality won't change


Screenshots:
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Campfire Single
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![campfire_single](https://f.cloud.github.com/assets/596076/1568217/4919b48a-50ab-11e3-8e74-25a8fbd26a01.png)

Campfire Multiple
-----------
![campfire_multiple](https://f.cloud.github.com/assets/596076/1568226/7f2f5b10-50ab-11e3-8450-421578fef54d.png)

Flowdock Single
-----------------------
![flowdock_single](https://f.cloud.github.com/assets/596076/1568229/8cec36f6-50ab-11e3-9c89-50c45f0c7e98.png)

Flowdock Multiple
--------------------------
![flowdock_multiple](https://f.cloud.github.com/assets/596076/1568232/958f0d92-50ab-11e3-82fe-f47a5cc9ebd6.png)

Hipchat Single
---------------------
![hipchat_single](https://f.cloud.github.com/assets/596076/1568234/9e038be2-50ab-11e3-88a7-74ddc16d729f.png)

Hipchat Multiple
-----------------------
![hipchat_multiple](https://f.cloud.github.com/assets/596076/1568237/a379a1c4-50ab-11e3-9f17-79790bae366a.png)

Mail Single
----------------
![mail_single](https://f.cloud.github.com/assets/596076/1568238/aa715db4-50ab-11e3-8cc6-f39f60de94f8.png)

Mail Multiple
------------------
![mail_multiple](https://f.cloud.github.com/assets/596076/1568241/b01ea7e4-50ab-11e3-9211-027761b2a1e7.png)

Opsgenie Single
------------------------
![opsgenie_single](https://f.cloud.github.com/assets/596076/1568244/b6bfe1da-50ab-11e3-8039-96a8617b0498.png)

Opsgenie Multiple
--------------------------
![opsgenie_multiple](https://f.cloud.github.com/assets/596076/1568248/bce6edc4-50ab-11e3-8c91-4bb6fce6d5e9.png)

Pagerduty Single
------------------------
![pagerduty_single](https://f.cloud.github.com/assets/596076/1568249/c4a01cf2-50ab-11e3-97d7-1a5dc9137f8c.png)

Pagerduty Multiple
---------------------------
![pagerduty_multiple](https://f.cloud.github.com/assets/596076/1568251/cbd8b254-50ab-11e3-9055-93f678bfa6f2.png)

Webhook Single
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![webhook_single](https://f.cloud.github.com/assets/596076/1568252/d2360340-50ab-11e3-9927-80a9bbff1254.png)

Webhook Multiple
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![webhook_multiple](https://f.cloud.github.com/assets/596076/1568253/d7b33950-50ab-11e3-8d69-b8b046111386.png)