Add a FileSign class with detached signing capability

This commit creates an object that can create detached signature of a file.

Set @key_id to select the signing key.

Usage:
  require 'signing_server'
  t = FileSign.new
  t.key_id = "SOMEKEYID"

  t.detach_sign("file_to_sign", "output_sig_file.asc")

Will create the signature file "output_sig_file.asc" from the input file
"file_to_sign" using the GPG key "SOMEKEYID".

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Add requirements to README.md