Add description for "proxy" HTTP runner parameter and fix the HTTP runner so
this argument actually works.

Note: The way proxies argument works meant I needed to split it into two
parameters - `http_proxy` and `https_proxy`.
Rename "redirects" HTTP runner argument to "allow_register" and add a
description for it.
Fix a bug with not printing a parameter default value when using "action --help"
if the default value was falsy (e.g. False, 0, etc.).
Add a description for "cookies" HTTP runner argument and fix the type to object
so it actually works.
Fix an issue with runner registration - if a runner already exists in the DB, we
still want to update the DB object instead of skipping the update.

Previously, we would never update the objects in the DB which means that if we
updated runner definition this would never get reflect in the DB unless the user
manually cleared the runner_type_db collection.
Remove deprecated and unused version modules and use st2common.__version__
attribute as an authoritative source for StackStorm version.
Don't disable existing loggers.
Fix and update packs documentation section.
Fix rules engine version reporting.
Fix example - variables need to be escapes.

Also add a note about the need to escape the variables when using the CLI.
Add missing metadata file for example simple polling sensor.
Use a better trigger names.
Merge pull request #854 from StackStorm/misc_0.6_fixes

Various 0.6 bug-fixes