bst operations from within workspaces

[Daniel Playle <dan playle gmail com>]

Hey all,

`bst` currently operates relative to `project.conf` in the project directory. This means that when it comes to the current workflow for workspaces, one must switch between the workspace and the bst project directory as part of the development cycle.

To provide a smoother experience, I'd like to see `bst` also operate from within open workspaces. This would require some mapping from the workspace to the associated project directory.

To achieve this, we can provide an implicit mapping by opening workspaces within a subdirectory of the project directory by default (we could call this `bst-workspaces` within the project directory, for instance). In this case, we can simply walk up the tree until we find the project directory. 

For the case where a user does not wish to have their workspace reside within their project directory, we can read the location of the project directory from some environment variable to still afford the ability to invoke `bst` in these workspaces.

I'd like to know what the community thinks of this workflow. Thoughts?

Daniel