zcaps: robust parsing of capability-invocation header

[llorllale]

Task #101 will implement a naive approach to parsing the capability-invocation header on HTTP requests. The approach does not account for subtle differences in the value of the header when, for instance, the same header is present multiple times in a request, or if there is whitespace around some identifiers/values.

A big part of the problem here is that the syntax of the capability-invocation header is not defined. As a matter of fact, the header itself is not defined anywhere.