Create faraday deprecation class

[BobbyMcWho]

Description

This implementation warns in stderr similar to Gem::Deprecate, but gives a semver instead of a date like the default Gem::Deprecate does.

require 'faraday/deprecate'

class SomeFaradayClass
  # deprecate instance method
  extend Faraday::Deprecate

  def old_method; end
  deprecate :old_method, :new_method, '1.0.0'

  def new_method; end

  class << self
    # when deprecating a class method, you must extend from within the eigenclass
    extend Faraday::Deprecate

    def old_class_method; end
    deprecate :old_class_method, :new_class_method, '1.0.0'

    def new_class_method; end
  end
end

Relates to #1052 #1043

[technoweenie]

This looks great, and takes care of the Error class stuff that I wanted to backport in #1052.

[technoweenie]

I think this should accept a version argument so we can proxy deprecated classes beyond 1.0. Probably not necessary for this PR on the 0.1x branch, but definitely once this gets ported to master.