Fixes #65

If specified, the command will run in non-interactive mode.

`bst workspace {open,close,reset} <bst> --source <index>` commands currently
give an indexing error. The python click library needs to be told that the
expected argument is of type integer.

This patch also makes use of metavar to display a placeholder in the help
prompt, like:

  -s, --source INDEX  The source to create a workspace for. Projects with one
                      source may omit this

Remove the requirement to specify '--force' in conjunction with
'--no-checkout' if there are already files in the workspace. We won't
write anything when opening the workspace, so there's nothing to force.

For example, when opening a workspace to an existing clone of a
repository, it seems alarming to have to '--force' the workspace open.
It made me wonder if it will actually be overwritten.

Fixes #60

Packaging is a big topic in the Python community these days. Things are
evolving, but a consensus seems to have formed around the path forward.

With PEP 518, Pip is becoming the primary tool to install Python
modules. In turn, Pip will use the right underlying tool for the job.
(distutils, setuptools, flint, ...)

Given all this, it makes sense to have a pip element in BuildStream.

This element installs a single Python module, telling Pip not to go and
download its dependencies, to make builds reproducible and not rely
on the network during builds.

By default it will use the `pip` command which generally points to Pip
for Python 2. Users can override the "pip" variable, for example to use
the `pip3` command, which generally points to Pip for Python 3.

This seems to be a simple copy-paste mistake.

To build all elements, the source bundle has to include the sources and
scripts for all elements, not just build dependencies and their runtime
dependencies.

This removes the --deps option as it doesn't make sense in this context.