Updates to the FTP deleteDir function

Hi

Currently the implementation of deleteDir assumes that the ftp_rawlist returns with an array with all files at the end. However, that seems not to be the case for all FTP servers. I have tested both on a synology FTP and pureftp server. 

I have changed the function to be recursive instead of relaying on the server to return the array correctly. 

Also the ftp_rmdir throws an exception when the file it tries to delete does not exist. I have changed the code to return false instead, to be aligned with the other specification.

PS: I am sorry if I mess something up, this is my very first pull request. 
No extra line here please.