Empty test cases for interior and exterior levels

Commit references #41.
Write unit tests for InteriorLevel(Level)

* Fill in unit tests for InterriorLevel class and it's parent.
* Add TestTile, TestStationary and TestStationaryBlocking mock objects.
In the future creation and usage of mocks is preferable when writing
tests.
* Improve InteriorLevel.get_nearest_players_coords method to show only
visible players.

Commit references #41.
Unit tests for ExteriorLevel

* Tests include some spaceship and projectile interaction.
* Projectiles now blow up when hit each other.

Commit references #41.
Empty unit test for bresenham and misc modules

Commit references #41.
Unit tests for game world setup

Commit references #41.
Unit tests for misc spaceship actions

Commit references #41.
Unit tests for Bresenham's get_line algorithm

Commit references #41.
Write tests for some player actions

Test for:
* activate_obj
* add_player
* all inventory and equipment actions

Commit references #41.
Finish unit tests for player actions

Wrap up tests:
* look
* move and attack
* interior fire

Commit references #41.
Test for TeleportPlatform class

Commit references #41.
Remove doctests and clean up

* Some PEP8 here and there.

Commit closes #41.
Merge branch 'unit-tests' into develop