Add support for Str node types

This adds support for AST nodes of type Str.  These nodes are simple
strings as defined in the source code, and have an 's' attribute that
contains the value of the string.
Minor PEP8 fixes
First test targeting Str nodes (binding to all interfaces)

This is the first test to target AST nodes of type Str.  It simply warns
on the existence of any strings matching "0.0.0.0", which indicate a possible
binding to all network interfaces of a network service / listener.