Tearsheet cleanup

removed a bunch of never-used styles like `.day-of-week`, `.tearsheet-add`, and `tearsheet-expandable`. These would be easy to re-build if we find a need for them. Also cleaned up the styles to be a little more similar to [Whitney tearsheet mockup](https://github.com/meetup/sassquatch/pull/120#issuecomment-34906076).

before:
![before](http://i.imgur.com/EBdCK7Q.png)

after:
![after](http://i.imgur.com/BHDdrMW.png)