Refactor ruby client classes and mixins

No functional changes, moves all client mixins to
lib/omega/client/mixins directory. Also extracts some
reusable operations from client entities into their own mixins
and refactors TrackEvents mixin by introducing internal helper
methods.

Finally introduces seeks_target mixin and destroyer entity
class which uses it (work in progress).
Set default ship/station system from block in client dsl

If 'ship' or 'station' is invoked within a 'solar_system'
block it will be created in the specified system (unless
explicitly overriden)
Syntax fixes, no functional changes
omega-cosmos-retrieve enhancements

Adds options to only display ships/stations that are alive,
to display extended attributes, and to show/hide callbacks
and notifications. Cleans up some output for conciseness.
examples/patrol.rb - few small formatting / spelling changes
Ship#attacking set/clear/check attacking_id attribute

'attacking_id' is passed in json / will always be present
if ship is attacking, where as 'attacking' foreign reference
may not be. Always check id attribute for consistency across
the board
Support coordinate/distance based auto-stop in linear strategy

Adds a 'stop_near' attribute to linear ms which if specified will
cause strategy to stop location movement when within the
specified distance of the specified coordinate (attribute should
be set to array containing distance and target x,y,z coordinates)

Uses this in lieu of stop_distance in manufactured
move_ship_in_system handler
Invoke command 'stop' hook when stopping entity commands

stop_hook allows command to cleanup / wrapup operations when
being manually stopped
Refactor Attack Command

Mostly style fixes though contains a few functional changes
including updating the registry attacker entity when attack
is stopped and changing ordering of callbacks invoked.

Also implements stop_hook to cleanup attacker operation
when command is stopped
Do not clear client entity handlers in destroyed state

Allows client to handle any remaining entity notifications that
come in. May want to add a delayed cleanup or similar eventually