less memory-intensive signing and verification of large files.

This is used in future code to sign zip files.  Signing the entire (base64-encoded) file at once was too memory-intensive.  

Solution proposed here is to sign chunks of the file, then make a signature file with all of the chunk signatures joined by newlines.  Verification proceeds with the same procedure.  Chunk size is a parameter to the functions.

Is this solution fair?  @jamalex any concerns?

Testing:
* Tested this by calling verification from the signing function directly--it worked.