Added client methods Mkdir() extend Remove for directories

Adds the Mkdir() to create a new directory on the SFTP server and also extends
Remove() to remove directories as well.
Fixed an error in marshalUint64() that resulted in incorrect marshalling of values >= 2^24
Added function Chtimes() to modify access and modification time of a file
Merge pull request #7 from nemith/master

Mkdir and remove directory, Chtimes, and marshal error