Several improvements to the HTTP DCG.

* [HTTP_DCG,DEB] Added http_to_gv/1 for exporting HTTP parse trees.
* [HTTP_DCG,SC] Unified the setting and reason arguments into the compound term setting/2.
* [HTTP_DCG,EXT] Added response//5 to the public predicates.
* [HTTP_DCG,EXT] Added the DCG rule for the 'Allow' header.
* [HTTP_DCG,DEB] Include the carriage-linefeeds and spaces in the parse tree as well (to inspect whether the specification is followed in every detail).
* [HTTP_DCG,DEB] Added major/1 and minor/1 subtrees to HTTP version's parse.
* [RADIX,NEW] Added number_to_digits/2 for splitting an integer up into a list of integers between 0 and 9.