Add validation capabilities over fetched contents

[jmfernandez]

Today I have found the scenario where some content fetched from FTP was corrupted through the download process. There are several validation mechanisms which can be integrated into WfExS-backend:

• When a file is a known compressed archive (tar, gz, bzip2, xz, zip), its integrity should be checked.
• When a file is signed, and a public signing key is available, check the file was not tampered.
• Declaring a file to be fetched containing MD5 or SHA1 sums or signatures of the fetched contents.
• Declaring inline fields containing the MD5 or SHA1 sums of the fetched contents.

[jmfernandez]

An initial validation of local integrity has been added at 5db1b8f