Use `Thread.current` in the Curb client to prevent `Curl::Err::MultiBadEasyHandle` errors

A temporary fix based on <https://github.com/celluloid/celluloid/wiki/Thread-safety-notes>

See eg. <http://blog.carbonfive.com/2011/10/11/a-modern-guide-to-threads/> for info about threads in Ruby

Example code:

    $LOAD_PATH.unshift File.expand_path('../../lib', __FILE__)

    require 'tire'
    require 'tire/http/clients/curb'

    Tire.configure { client Tire::HTTP::Client::Curb }

    class Book
      include Tire::Model::Persistence
      property :title
    end

    Book.tire.index.delete

    Book.create title: 'Foo'
    Book.create title: 'Bar'

    Book.tire.index.refresh

    threads = []

    %w| foo bar |.each do |q|
      threads << Thread.new do
        p Book.search( q ).results.to_a
        puts '-----'
      end
    end

    threads.each { |t| t.join() }

Closes #144