Create correct << method that returns self instead of total_written

Consult http://rdoc.info/stdlib/core/IO#%3C%3C-instance_method vs. http://rdoc.info/stdlib/core/IO#write-instance_method
Merge pull request #38 from bkerley/patch-1

Create correct << method that returns self instead of total_written
Fix usage of legacy async syntax in benchmarks
Allow failures on jruby-head