Added Client_Mtime property to MetaData class

Added support for the "client_mtime" field in Dropbox API (I don't know since when it exists though).

Added properties:
1. string Client_Mtime
2. DateTime Client_MtimeDate
3. DateTime UTCDateClient_Mtime

For 2 and 3, I moved the code out from ModifiedDate and UTCDateModified and created separate private static methods so that both Modified and Client_Mtime could use the same code.

I'm doing the pull request for the first time, but I hope I'm doing it correctly.

Thanks!