search handler: all funcs now rid of manual maps responses.

serveDescribe, servePermanodesWithAttr, and serveSignerPaths
rewritten like serveRecent. Got rid of populateJSON.

http://code.google.com/p/camlistore/issues/detail?id=101

Change-Id: Id730d40005476634b138b13260248e497d1a87d0
Add pkg/types with types.Time3339; use it in search/handler JSON types

Change-Id: I79bf1484ef8b3548bca2e732f189352bcfbcd6db
httputil/handler: start of more formal approach to input validation and HTTP error handling

Change-Id: I219a6263e1ba2d0e64819c01a885078dbf1c8b4c