Pass activities and checksums to FTS3

[marcmengel]

This is a draft implementation of issue #3 . It currently needs testing, particularly with an actual FTS3 instance,
but it should let you:

• specify an "Activity = xxx" in the Mover section of the config that will get passed to the FTS3 on transfer requests
• pass Adler32 checksums in metadata (in old format, SAM format, or Metacat format) to the FTS3 for transfer validation.