Extra initializers to configure disk cache prefix.

Two extra initializers, one on TMCache and one of TMDiskCache which accept
a prefix, as well as a name, allowing the prefix to be configured just like the
name is. The existing API is untouched - `initWithName` will use the existing
default prefix.

The tests pass, but I haven't added any specifically for this, or regenerated the docs, so this probably needs doing if you're happy to accept the pull request.

Thanks!