Took out initialize_balance

I felt that the name alone implied this should probably be in the initialize method.
It did lead to some horrible instance_variable_set calls but then I blame UnitTests limitations for that and it's better than unnecessarily opening up balance as an attr_writer
Refactoring remove player method, not sure it worked before!
Some minor refactors
Refactoring the engine slightly, just playing around really but found the winner throw/catch was confusing
Use hashes rather than loads of separate constants
Don't need .rb in require statements
Add owned by player
More removing .rb in require
Merge pull request #2 from maker-leo/monopoly-refactor

Monopoly refactor