Fleets:
1) added some logic for handling resource-collection missions in Fleet.php under method destination_reached().

World Objects:
1) Added method "yield_resources" to some children of the world_object class (asteroids and wreckage).
2) Added a default method "yield_resources" to the world_object class that returns no resources and acts as a default case for children that don't implement this method.
Merging with Brian's changes.
Database:
1) Created a table, `fleet_cargo`, to keep track of the on-board cargo for fleets.
2) As per brian's code updates, the `world_object` table has been modified to include two new fields: mass, building_type

War Engine:
1) Fixed an issue with scouting not getting completed properly due to recent changes to the world objects' classes.
2) Finished coding logic that lets fleets with "collect_resources" mission extract resources from a world object.
3) Added logic that adds a returning fleet's cargo to the home-colony's resource pool.

misc:
1) Fixed some silly typos in my previous, un-pushed, commits.
2) Changed command center content file to save resource updates to the database.

==NEXT UP==
1) Add an option from the map interface to send ships on resource collection missions.
Updated the lygos.sql file in this project folder to reflect changes to the database. Simply run this sql file in your phpmyadmin after selecting the lygos DB by clicking 'import' and selecting this file.