NO-JIRA: Fix ignored python exceptions "Selectable' object has no attribute '_impl"

Fix errors introduced by:
  e769ad5 NO-JIRA: Fix core dumps and memory management errors in  python binding, engine.py.
Deleted attribute _impl was later being checked for None.
PROTON-772: Avoid use of GNU extension ## command deletion.

The following idiom is GNU C specific:
-#define pn_logf(fmt, ...)
-            pn_logf_impl(fmt , ##__VA_ARGS__);

The use of ## after a comma deletes the comma if __VA_ARGS__ is empty, so the above works
even if there is no argument after fmt.

The following is more portable:

+#define pn_logf(...)
+            pn_logf_impl(__VA_ARGS__);

Now fmt is included in the variadic argument list so it is never empty.  This is
slightly less self-descriptive since it doesn't correspond as well to the
signature of pn_logf_impl(fmt, ...), but it is standard C99.
added log.c to javascript build
PROTON-774: Fix warnings in log.c and url.c and re-enable -Werror in JavaScript CmakeLists.txt
NO-JIRA: fix wrapper.py not included in python install
PROTON-779: Updated Python docs generation to use the new source files.
Updated after removal of the released property on Delivery
PROTON-697: SChannel functionality: Proton-specific trust stores, client certificates
PROTON-780: Fix "variadic macro" compiler errors under GNU C++ compiler.
PROTON-772: New proton logging code

Based on list discussion:
- moved the log statement api into src/log_private
- simplified the public API to 2 functions: pn_log_enable and pn_log_logger

The minimal public API is required to let applications using proton control
where proton log output goes, which is the point of all this.

The present state of the logging achieves the following goals:
- allow applications to disable or redirect ALL proton library output, nothing is forced to stderr.
- leaves the previous behavior of transport tracers unaffected.
- introduces as little disturbance as possible (one new env. var PN_TRACE_LOG and 2 new API functions)

This is definitely not the final word on logging for proton, but a more
sophisticated, unified logging system requires more discussion in the proton
community.
added link level redirect to pn_condition_is_redirect
PROTON-471: Add Messenger->work to Perl bindings

Added methods for checking that a Messenger instance is blocking. Also
added asynchronous examples for Perl.

Added a recv_async and a send_async example app for Perl, based on the
Python apps.
PROTON-471: Example for Messenger->Work in Perl.
PROTON-471: Reworked how Perl processes error results from API calls.

It does not die on qpid::proton::Errors::INPROGRESS.
PROTON-471: Removed the bool mapping for arguments in the Perl bindings.
added a script for setting the version of the tree
updated release script for git
added export script
use the current branch instead of hardcoding origin/master
added instructions for pushing