storage/http: Use URI Templates for configuration

Rather than using our own custom URI template syntax, this commit brings
the configuration syntax in line with the evolving "URI Template"
standard found in RFC6570.

Only Level 1 URI template support is implemented here: the caveat is
only that IDs should contain URI-safe characters only.

For more about the various levels of support for URI Templates, see

    http://tools.ietf.org/html/rfc6570#section-1.2

Closes #328.