Implement tag handling

[pdl]

It should be possible to tag test cases with strings, e.g.requires_connection, so that they can be skipped.

The TestRunner would then be configured with a blacklist of tags to ignore.

• Are all strings permissible, or do we have a constraint like /^[a-z0-9_]+$/i?
• Are tags case-sensitive?
• Will we ever want tags with key-value pairs or some other structure?

[pdl]

b3ef0f2 on the Tags branch completes the implementation of this.