Hide all the pbuilder internals

With this patch plainbuilder has just one command:
	plainbuilder build file.dsc
which does everything else needed internally.

There is some provisioning for supporting multiple different versions
of Ubuntu later but so far only 12.04 (precise) is being used

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>