Skip tags in Zend_Cache for Aggregate if not available

Allows usage of one consistent Zend_Cache backend
for all SilverStripe core storage, greatly simplifying
its configuration. This means a call to Aggregate::flushCache()
will indeed flush all caches if the backend doesn't support tags,
including any custom caches defined through SS_Cache.
Given caches are regarded transient that's an acceptable limitation.
Improved SS_Cache docs, moved to /docs

We keep having trouble with extended PHPDoc formatting,
and have similar topics covered in Markdown already.
Moved docs, added some details about timeouts and
the theory of caching backends.
Merge pull request #2677 from chillu/pulls/zend-aggregate

Skip tags in Zend_Cache for Aggregate if not available, better docs
Merge remote-tracking branch 'origin/3.1'