Collection export

Hello!

I am so used to export and import of individual collections, I thought mongo-express should have it! Right now I've added export and it seems to work fine! If you like it and think it could part of this project please accept this pull request!

Some screen shots:

**Collection listing page:**
-

![exp-1](https://cloud.githubusercontent.com/assets/1248984/3347654/81d80256-f8f1-11e3-93f7-413c0e83881b.png)

**View collection page:**
-

![exp-2](https://cloud.githubusercontent.com/assets/1248984/3347653/81d7e424-f8f1-11e3-9958-526e18a266f2.png)


Thanks!