Add a Scanner class that finds ROP Gadgets

[postmodern]

Add a Scanner class which takes a .text section object or IO stream and uses ronin-asm's disassembler(s) to disassemble each instruction, then looks for instructions prior to each ret instruction (or equivalent instruction for the architecture of the .text section/executable file). It should yield each discovered Gadget object (see #7).

executable_file = ExecutableFile.new('/bin/ls')
scanner = Scanner.new(executable_file, ...) # additional configuration options may be given

scanner.each do |gadget|
  puts "Found gadget at offset #{gadget.offset}"
  puts
  puts gadget.instructions
end