This change adds support for PROPFIND requests to net/webdav.
It contains a proposed PropSystem interface and a preliminary
implementation of an in-memory property system. As discussed
with nigeltao, this is the first of approximately 4-5 CLs to
get property support in the net/webdav package.

Current coverage of litmus 'props' test suite:
16 tests were skipped, 14 tests run. 10 passed, 4 failed. 71.4%

Change-Id: I0bc5f375422137e911a2f6fb0e99c43a5a52d5ac
Reviewed-on: https://go-review.googlesource.com/3417Reviewed-by: Nigel Tao <nigeltao@golang.org>