Adding support for subquests in the Inform7 code.

[MarcCote]

This PR follows #42 and contains the second and final modification that will bring support to generate games with multiple subquests. It supersedes the rest of PR #30.

This PR adds support for subquests for different parts of the framework.

• Inform7 code: adds code for checking the winning conditions of the different subquests, when all conditions are being met increase the score by some amount defined by the reward attribute of Quest objects.
• Text generation: makes the text generation for quest description more flexible.
• Game: The objective of the game is now stored in the Game objects. It is generated from the main_quest (a Quest instance) which is created by merging all the subquest dependency trees and extracting a winning policy from the resulting ActionDependencyTree object.
• Unit tests: some tests needed to be adapted to include the max/min_breadth information. Also, adds more tests for testing the new code.

[tavianator]

Looks good! I made a couple suggestions, up to you if you want to integrate those in this PR or leave it for later.

[tavianator]

Well, also fix this test I guess:

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FAIL: test_world2inform7.test_names_disambiguation
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Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/travis/build/Microsoft/TextWorld/textworld/generator/inform7/tests/test_world2inform7.py", line 165, in test_names_disambiguation
    assert "tasty apple" in game_state.inventory
AssertionError

[MarcCote]

The test is now passing.