SFTP: add file_exists, is_dir and is_file functions

also expand caching layer
SFTP: add support for recursive nlist and rawlist
SFTP: switch from using file existence cache to stat cache, like PHP

also add a few new functions - is_link and filesize
SFTP: use_cache -> use_stat_cache
Merge pull request #338 from terrafrost/sftp-changes-20140510

SFTP: add file_exists, is_dir and is_file functions

* terrafrost/sftp-changes-20140510:
  SFTP: use_cache -> use_stat_cache
  SFTP: switch from using file existence cache to stat cache, like PHP
  SFTP: add support for recursive nlist and rawlist
  SFTP: add file_exists, is_dir and is_file functions