* add site_config_dirs() to appdirs to determine locations across OSes
* add system_config_files to locations.py
* add system_config_files to get_config_files() and re-order files entries to correct precedence
* document changes to configuration files in user guide
Closes#309
* Deprecates the --download-cache option & removes the download
cache code.
* Removes the in memory page cache on the index
* Uses CacheControl to cache all cacheable HTTP requests to the
filesystem.
* Properly handles CacheControl headers for unconditional
caching.
* Will use ETag and Last-Modified headers to attempt to do a
conditional HTTP request to speed up cache misses and turn
them into cache hits.
* Removes some concurrency unsafe code in the download cache
accesses.
* Uses a Cache-Control request header to limit the maximum
length of time a cache is valid for.
* Adds pip.appdirs to handle platform specific application
directories such as cache, config, data, etc.