Add multiple revision support to stamp, support purge

Add multiple revision support to stamp, support purge

Added new ``--purge`` flag to the ``alembic stamp`` command, which will
unconditionally erase the version table before stamping anything.  This is
useful for development where non-existent version identifiers might be left
within the table.  Additionally, ``alembic.stamp`` now supports a list of
revision identifiers, which are intended to allow setting up muliple heads
at once.  Overall handling of version identifiers within the
``alembic.stamp`` command has been improved with many new tests and
use cases added.

Fixes: #473
Change-Id: If06501b69afae9956df3d0bcd739063fb8042a02