Handful of performance optimizations

Could use a second (third?) pair of eyes on these.  Hence the pull request instead of a direct commit.

Here's the benchmark @richievos and I have been using (more-or-less)

    require 'benchmark'
    require 'cgi'

    def benchmark(msg, iterations=1000, &block)
      puts "#{msg} (n=#{iterations})"
      GC.start
      #RubyProf.start
      times = Benchmark.ms do
        iterations.times(&block)
      end
      #profile = RubyProf.stop

      puts "\ttime: #{(times / iterations.to_f).round(2)}"
      #printer = RubyProf::GraphPrinter.new(profile)
      #printer.print(STDOUT)
      #printer = RubyProf::CallTreePrinter.new(profile)
      #printer.print(File.new('/var/tmp/rubyprof.output', "w"))
    end

    def run
      puts "===Typhoeus version: #{Typhoeus::VERSION}==="
      hydra = Typhoeus::Hydra.new
      benchmark("making couple requests") do
        request = Typhoeus::Request.new("http://internal-url.company.com?with=params", :method => :get)
        hydra.queue(request)
        hydra.run
      end
    end

    run

Before (pasting one sample run of many):

    ===Typhoeus version: 0.6.2===
    making couple requests (n=1000)
    	    time: 3.02

After (one sample of many similar results):

    ===Typhoeus version: 0.6.2===
    making couple requests (n=1000)
	    time: 2.02