Revised #26 (use elasticsearch-ruby and "patron", handle multiple nodes with load balance, add utc_index option, fix bugs, etc)

At first, thanks for @xia0pin9 to provide base implementation. Without this I would waste lots of time. :)

It's revised version of PR #26 with unit test & real run passed.

+ include handling with multiple hosts with load balanced

Change Log
====

- bug fixes
  - meta/record are not sent to ES when bulk_message.size grows and equals to @flush_size.
  - some typos
  - ES 1.x may request ping (request http://<ES>:9200/ with HEAD method), so we should stub it
  - client.bulk accepts array of hash, not json

- improvements
  - now we can handle multiple hosts with load balanced!!
    - load balance method : round-robin (elasticsearch-ruby's default strategy)
    - just use new option : hosts, describe host:port list separated by ","

- library dependencies
  - use patron, recent typhoeus (0.6.7) is not compatible with recent faraday (0.9.0)
    - you need to install libcurl

- etc
  - arrange whitespaces for proper formatting

If we need to avoid installing libcurl, we can just test another drivers to match or disable keep-alive feature (by not installing typheous and patron, remove dependency) and go on.

I've changed source code too often, so you may think commits are dirty.
Please feel free to ask me, I'm willing to clean up and merge commits if needed.

Thanks!