DescriptorExample, URLUtil & Example directory http requests

DescriptorExample: requests tor router descriptors …
    * consensus, numeric random, and binary random
    * random directory (cache), and fixed directory cache
	- add functions to Consensus to permit fixed directories
    * URLUtil for encoding and decoding URL percent-escaping
        - used by DescriptorExample to preserve non-printable bytes

src/test/examples: sample http requests to a tor directory, including:
    tor_log_bell.http: a request which raises a BEL in the server tor log
    		       (requires debug mode and a compatible terminal)
    See the README for more details

DIRECTORY_URL_SPEC: minor clarifications and typo fixes