Make file operation work with CSV library

[jstuckey]

This is to fix the issue reported in #65.

The two missing pieces of the IO-like object needed by the CSV library are a limit argument for the #getsmethod and a #rewind method. I've attempted to match the interface described by Ruby's IO class.

The flexibility of arguments that #gets accepts makes for some messy conditional code. Hopefully the changes I've made are clear. According to the docs, the first argument can have the following behavior:

• Separator string: read until separator is found
• Zero-length string: read until end of paragraph
• Integer: read N bytes
• Nil: read until end of file

These can also be paired with a second limit argument.

I've added some tests and also tested with the following code:

require 'csv'

$LOAD_PATH.unshift "#{File.dirname(__FILE__)}/lib"
require 'net/sftp'

Net::SFTP.start("my_hostname", "my_username", password: "my_password") do |sftp|
  stream = sftp.file.open("my_filepath.csv")
  CSV.new(stream).each { |row| puts row }
end