Add Signature, File, Validator modules

* All these module require some more work to be of any kind of use.
Make Filemagick::Signatures a singleton
Add validation logic
Add basic validator code. Add some specs
Rename 'read_file!' method

* This method doesn't actually read anything, just instantiates the IO
  object if a path is passed or, if an IO object is passed, returns the
  rewinded IO object.
Use filemagick-signatures gem; remove signature code
Add basic specs for pdf and jpeg files
Add filemagick-signatures to Gemfile

* This will enable easier development of both the gems simultaneously
  without releasing newer version of either.
Add spec file File class
Gitignore vendor directory
Add rspec config file to VCS