Add tracking for mtime/file hashes in metadata file

[sboesen]

Will fix #70 and #40.

TODO:

- Add setting to enable feature file hashing comparison

• Compute hash locally on sync only under certain conditions (remote doesn't have file, remote has file that local doesn't, etc)
• Compare hashes when determining action for files
• Save hashes in remote metadata
• Add mtime in generic file
• Change mtime comparisons from platform-specific code to this
• Remove option to disable metadata sync in S3

Also a reminder to remove S3 cloud provider specific warnings in readme

[kadisonm]

Looking forward to this!