e4c support

During development, several issues with librabbitmq triggered
segmentation fault. The librabbitmq code doesn't safeguard
against null pointer errors.

To mitigate this behavior in environment where it's critical a
crash of librabbitmq doesn't propagate to the TCL interpreter,
then to the host application, we use the e4c library to catch
these errors as exceptions.

Import of e4c library

Source: https://github.com/guillermocalvo/exceptions4c
Commit: 17594e9
Version: 3.0.5

Modifications done to our extension code

• We introduce a USE_E4C compiler define to enable/disable e4c code
• e4c context begins during initalization code
• we add unload code to end the context
• mq_command is wrapped in a try/catch block