changed the task modal start/complete button colors

This PR resolves #150. Specifically, I made the following three changes to the start/complete button on the task modal: 

1) The start button is now blue: 
![image](https://cloud.githubusercontent.com/assets/5275384/5459315/df764d1a-850e-11e4-8272-4a9c65a8469e.png)

2) The complete button (once task is started) is green: 
![image](https://cloud.githubusercontent.com/assets/5275384/5459311/d65ac710-850e-11e4-94a8-cc7bcb213197.png)

3) When the task is completed, the button is a faded out green (since it is disabled): 
![image](https://cloud.githubusercontent.com/assets/5275384/5459302/cba4b754-850e-11e4-99ab-d3117349969d.png)

When testing, create a team, start it and make sure the buttons are the right color. Might want to check in both worker and author view to make sure the changes persist but they should :)