add forceFlushCache()

This ignores any logic added to flush the cache intelligently and instead just flushes all available namespaces. This makes code simpler when we know we want to flush everything at the cost of having to make getCacheNsArray() respect a new $ignore_checks flag in each dataobject.
Cleanup of namespace caching.

Thanks to @gauthierm, this is at least 1% better.

Rename methods:
 * getCacheNsArray() to getCacheNamespaces()
 * getFullCacheNsArray() to getAvailableCacheNamespaces()
 * flushCache() to flushCacheNamespac()
 * forceFlushCache() to flushAvailableCacheNamespaces()

As well update behaviour of getAvailableCacheNamespac() to return a flat array of namespaces to flush, instead of trying to build that logic into getCacheNamespaces().
line-length
Documentation cleanup.
Merge pull request #20 from isaacgrant/better-force-flushing

More robust cache flushing methods.