Fixes #26: Attach 'select'

I could only get the ffi_lib calls to perform properly when included in functions.rb, like @brainsucker did in 59e9505. It might not be as pretty, but it works.

**Before:**
```ruby
irb(main):001:0> require "ethon"
FFI::NotFoundError: Function 'select' not found in [libcurl]
from D:/ruby/lib/ruby/gems/1.9.1/gems/ffi-1.7.0.dev/lib/ffi/library.rb:251:in `attach_function'
from D:/ruby/lib/ruby/gems/1.9.1/gems/ethon-0.5.11/lib/ethon/curls/functions.rb:60:in `extended'
from D:/ruby/lib/ruby/gems/1.9.1/gems/ethon-0.5.11/lib/ethon/curl.rb:47:in `extend'
from D:/ruby/lib/ruby/gems/1.9.1/gems/ethon-0.5.11/lib/ethon/curl.rb:47:in `<module:Curl>'
from D:/ruby/lib/ruby/gems/1.9.1/gems/ethon-0.5.11/lib/ethon/curl.rb:19:in `<module:Ethon>'
from D:/ruby/lib/ruby/gems/1.9.1/gems/ethon-0.5.11/lib/ethon/curl.rb:14:in `<top (required)>'
from D:/ruby/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from D:/ruby/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from D:/ruby/lib/ruby/gems/1.9.1/gems/ethon-0.5.11/lib/ethon.rb:8:in `<top (required)>'
from D:/ruby/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `require'
from D:/ruby/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `rescue in require'
from D:/ruby/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:35:in `require'
from (irb):1
from D:/ruby/bin/irb:12:in `<main>'
```

**After:**
```ruby
irb(main):001:0> require "ethon"
=> true
irb(main):002:0> require "typhoeus"
=> true
```