concept of caching output context

- moved caching methods to the CacheManager class
- comparing context dicts does not work well,
  because Article, etc. classes don't have
  __eq__ implemented
add Content__eq__ and handle tag_cloud as a set

thereby output_context caching appears to work
improve Content__eq__ dosctring
move output context caching to _write_file

- pagination can change
- path joined only once
Merge branch 'generate_only_modified-cache_output_context' into generate_only_modified

Conflicts:
	pelican/generators.py
	pelican/settings.py
GZIP_CACHE setting to speed up IO
refactoring: CacheManager -> File{,Stamp}DataCacher

move common code into reusable classes
document output context caching settings
document GZIP_CACHE changing troubles