CRUD Interface to manage volunteer availabilities

![volunteer_availability_main](https://cloud.githubusercontent.com/assets/3430891/4507626/1df8af00-4b0e-11e4-853c-6601dd481da0.PNG)
![volunteer_availability_modal](https://cloud.githubusercontent.com/assets/3430891/4507627/1df90d38-4b0e-11e4-847b-28a5fb65d812.PNG)

Modal is triggered on Add and Edit
Delete should trigger confirmation box.


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we'll have to change this to use the TableSorter after #126