War Engine:
1) Moved computation of travel duration into the method "travel_time" in the class "Fleet" so that the calculations will be consistent.
2) Added a functional "hold position" option to the sector actions menu.
3) When a fleet reaches its target, it performs an action based on its mission and then returns home (unless its mission was to hold the position, in which case it stays there).
4) Debugged the fleet-battle mechanics in class "Mega_Fleet".

Database:
1) Changed field `speed` in table `ships` to be datatype float instead of int. This will allow for cleaner, more flexible travel computations.
2) Added field `player_id` to table `jobs` for jobs that transcend colonies such as fleet movements.
3) Changed some coordinate fields in the database to be signed. We need those negative coordinates!

Misc:
1) Updated ship graphics.
2) Moved style for job queue mini div into a stylesheet.
3) Fixed a bug where duplicate event listeners were attached to fleet-dispatch buttons.
4) Updated evasion and accuracy values for the four basic ship types.
5) Updated job queue to work better with fleet movements.