Add '(De)Select all' buttons to the Metadata Curation page.

Closes #536 

It wasn't specified but I gave a min-width to the button otherwise the columns bounce around as the text changes from `Select all` to `Deselect all` since the button text has different lengths.

![metadata-admin](https://f.cloud.github.com/assets/96776/2480298/c3c9ba76-b0b5-11e3-91ad-5ce3b5f33c9d.png)