Install build-essential with mongodb

When working with mongodb many users will also need to install the c++ bson library (either through npm or other means)

For this extension to properly install, `build-essential` (along with make and gcc) are required.

Otherwise everyone is left in the cold with a slow native js version of the bson interpreter!

And no one likes things that are slow!

Slow BSON makes people :arrow_down: 
![Slow is bad](http://mashable.com/wp-content/uploads/2013/07/Dr.-Who.gif)