Remove the ability to run multiple commands

Addresses #1747.
This removes the argument list iteration and spawning of multiple
tasks.
It's still possible to specify aditional arguments but now they're
ignored.

PR: #1752
spec the absense and wrong commands

PR: #1752
stringify all symbols in twitter hash


Fixes #1450
add robustness to tweets stream handling


Fixes #1450
monkey patch twitter gem to trap json parsing exceptions and use our own json parser


Fixes #1450
support :symbolize_keys options for json parsing


Fixes #1450
honor common to_json method signature


Fixes #1772
add ruby filter spec


Fixes #1772
Updated link in syslog documentation


Fixes #1724
specify ctrl-d to halt pipeline


Fixes #1767
CTRL-C Enter is OK too


Fixes #1767
Merge branch 'master' of github.com:elasticsearch/logstash
Remove retry logic and just try to install template until successing as the first time