Added ClientMtime property to the Metadata

Added ClientMtime (to support the "client_mtime" field) to the Metadata.

This is the same type of modification I made to the DropNet few weeks ago (apparently I didn't know the existence of DropNetRT until recently! But now I've migrated to this library).

Added properties:
 1. string ClientMtime
 2. DateTime ClientMtimeDate
 3. DateTime UTCDateClientMtime

For 2 and 3, I moved the code out from ModifiedDate and UTCDateModified and created separate private static methods so that both Modified and ClientMtime could use the same code.

Thanks!