Various updates:

• #195 - Bugfix around STAT response code (by @drakkan)
• #196 - Added support for hash commands (by @drakkan)
• #197 - Exposing last received FTP command (by @drakkan)
• #199 - Bugfix around MFMT command
• #198 - Added AVLB command to provide the available space (by @drakkan)
• #202 - Bugfix around STOR command in case of error
• #203 - Added COMB command to combine files into one (by @drakkan)
• #204 - Allowing to disable SYST and STAT commands (by @drakkan)
• #205 - Added SITE MKDIR and SITE RMDIR commands