Remove an extra colon that caused array lookup to fail

Make the packaging script "shellcheck" clean

Enhanced stack-traces via backwards library

Backwards library provides basic stack-traces of a crash and enhanced
stack-traces by using libdw and/or libbfd.
This patch will detect if either library exists on the system and set
the appropriate compiler directives accordingly.
The libraries will be packages as part of the Lambda zip file.

Add User-Agent information (awslabs#11)

* Add User-Agent information

Send a user-agent http header when talking to Lambda's http server.
This is important for collecting usage metrics.