Add missing 'require "stringio"' in analyzer.rb

Example usage failed in my machine:

```
t = analyzer.tokens
 => #<Enumerator: #<Enumerator::Generator:0x007fad739716c0>:each>
1.9.3-p362 :007 > t.first
NameError: uninitialized constant FreeLing::Analyzer::StringIO
	from /Users/manuel/.rvm/gems/ruby-1.9.3-p362/gems/freeling-analyzer-0.1.0/lib/freeling/analyzer.rb:91:in `block in read_tokens'
	from /Users/manuel/.rvm/gems/ruby-1.9.3-p362/gems/freeling-analyzer-0.1.0/lib/freeling/analyzer.rb:62:in `each'
	from /Users/manuel/.rvm/gems/ruby-1.9.3-p362/gems/freeling-analyzer-0.1.0/lib/freeling/analyzer.rb:62:in `each'
	from /Users/manuel/.rvm/gems/ruby-1.9.3-p362/gems/freeling-analyzer-0.1.0/lib/freeling/analyzer.rb:62:in `block in tokens'
	from (irb):7:in `each'
	from (irb):7:in `each'
	from (irb):7:in `first'
	from (irb):7
	from /Users/manuel/.rvm/rubies/ruby-1.9.3-p362/bin/irb:16:in `<main>'

```

Seems that there was a missing require in  `analyzer.rb`. Hope this helps!