Add Pry console and simple readline command shell

Adds a blocking command shell reading input with readline, passing
to a simple query execution wrapper, and returning _raw output in
plain text.

TODO:
A proper REPL implementation with multi-format parsing and local
data storage/manipulation is required for serious endeavors at the
CLI level. Pry would likely be a good starting candidate for this.

Usage Example:
```
ruby 7_splunk_shell.rb 
splunk> search index=main | head 1

Thu, 4 Apr 2013 09:01:45 PDT title="Lieberman: Griner to Mavs has no downside"
link=http://espn.go.com/dallas/nba/story/_/id/9131784/nancy-lieberman-says-there-no-downside-brittney-griner-trying-make-dallas-mavericks
description="Women's basketball pioneer Nancy Lieberman is encouraging Baylor star Brittney Griner to seize the opportunity to do the same with the Dallas Mavericks, no matter how overwhelmingly remote Griner's chances of sticking actually are."
splunk> exit