added 'select member role' as the default in the dropdown for the role 

Two changes to the hiring emails: 

1) #35: The dropdown for selecting the member role in the task available, task acceptance and task rejection emails, now shows "-- SELECT MEMBER ROLE --" as the default [when testing, make sure that you can't send emails if you haven't selected a member]:
![image](https://cloud.githubusercontent.com/assets/5275384/5460498/482b9e06-8518-11e4-9c0b-de3600158f86.png)

2) #141:  I added "OPTIONAL" to the beginning of the placeholder text for the field with the input deliverable in the task available and task acceptance emails:
![image](https://cloud.githubusercontent.com/assets/5275384/5460489/2728c29c-8518-11e4-8ebf-69b8a7446621.png)