Fix bug + increase disk size

Update the README.md file to work better with github.

Don't warn in if LXC.

Fix --user in --help to display how the command actually works

Increase ssh timeout to 30 seconds

The previous 5-second timeout causes random errors during compilation
here, increasing it to 30 seconds solves this problem.

Don't install package recommends implicitly

Bugfix: Use noninteractive frontend for apt-get

Bugfix: Pass --update-head-ok to git fetch to avoid problems overwriting checked out branch

Perhaps also actually enable KVM...

Use qemu-system- instead of kvm -m

As kvm -m doesn't work in Ubuntu 13.04 anymore.

Allow environment-configurable KVM binary name

Bugfix: git fetch directly from the remote URL, rather than only setting a default the first time

Update README to include Gentoo instructions as well

- Fixes problem where the gitian URL varies for different branches of the same repository
- Explicitly fetching tags using + allows repository to overwrite existing tags (changed tag)
- Fetching branch heads to local heads correctly makes branch names available for commit option
- Fetching both branches and tags at the same time optimizes things slightly

Virtualbox support

Some packages use uname -m, which is not set to i386 on LXC on a 64 bit
host.

fixes #24

Make using an invalid tag/commit an error

Fix prereqs

Added ruby.
LXC and KVM both need python-vm-builder.
And apt-cacher never worked for me, but apt-cacher-ng does.