Add keep-alive support

I simply use `net/http/persistent` for easy keep-alive implementation

A little benchmark:
```ruby
require './rest-client'
require 'benchmark'

puts Benchmark.measure{ 10.times{ RestClient.get('http://www.google.com/').length } }
RestClient.persistent = true
puts Benchmark.measure{ 10.times{ RestClient.get('http://www.google.com/').length } }
```

I get those numbers
```
  0.080000   0.010000   0.090000 (  1.365598)
  0.030000   0.000000   0.030000 (  0.354387)
```

For now the integration is a bit "Quick & Dirty". It's just to show the eventual benefits.
But if you like the idea I can clean it up and add test coverage.

Regards,