Add mercurial to the list of packages needed in a dev environment

The dev environment setup procedure fails if mercurial is not installed
on the machine, it cannot download stuff from
http://code.google.com/p/go.net for instance.

README.md

@@ -329,7 +329,7 @@ Setting up a dev environment
 Instructions that have been verified to work on Ubuntu 12.10,
 
 ```bash
-sudo apt-get -y install lxc curl xz-utils golang git
+sudo apt-get -y install lxc curl xz-utils golang git mercurial
 
 export GOPATH=~/go/
 export PATH=$GOPATH/bin:$PATH