Fix longstanding issue with cache class & cachetime setting

1. cachetime setting should only be applied to the cache_renderer
   class.  Previously it was applied to cache_parser (and by
   extension cache_handler).
2. two special cachetime values, -1 & 0, weren't handled, per
   FS#2183

To handle the cachetime setting, -1, disable caching, a new
property _nocache is added.  When that property is true, any cache
file must not be used and storecache() should not store any values
put PHPDocs comment back the way it should be
adjust logic of caching comments for _nocache
Merge pull request #618 from splitbrain/cache_and_cachetime

Fix longstanding issue with cache class & cachetime setting