Configurable project name

[rdnt]

Closes #12

• If empty, set project name based on the project's absolute filesystem path
• Validate uniqueness of project name across aggregated bobfiles
• Project name passed to tasks, so that artifacts can be properly stored if needed (e.g. in a remote store)
• Playground generation slightly modified to help test the project name

[Equanox]

Closes #12

[rdnt]

@Equanox regarding TODO: maybe don't leak absolute path of environment (bob/aggregate.go:108):

Since we will not be syncing projects without a proper remote set up as a project name, there is no possibility to have an absolute path in a remote artifact's metadata. Let me know if I should remove the comment from the code.

[Equanox]

@rdnt yes that can be unexpected.. let's keep an eye on it.. though it seems to be save for now.