Make Promise subclassable using classmethods, add asyncio await support

[csboling]

These commits make the following changes to the code, reflecting them in the tests and docs.

• Promise.fulfilled/resolve, Promise.rejected, and Promise.all were converted from staticmethods to classmethods, allowing them to invoke constructors and other classmethods via cls() and cls.<method> rather than Promise() and Promise.<method>. Additionally class atttributes are now referenced using self.<attribute> rather than Promise.<attribute>. This improves support for classes which inherit from Promise, since if client code defines a subclass SpecialPromise then calling e.g. SpecialPromise.all or SpecialPromise().then() will now return a SpecialPromise instance rather than a Promise instance.
• promisify and promise_for_dict were made into classmethods rather than free functions for the same reason. To preserve API compatibility we also alias promisify = Promise.promisify and promise_for_dict = Promise.promise_for_dict. Parameterized test fixtures were added to run tests against both the classmethod and the free function versions.
• __iter__ and __await__ methods were added to let Python 3.4 and 3.5 code yield from or await a Promise in asynchronous code. This makes working with Promises feel more native when using asyncio and aligns with ES7's async/await behavior for native promises. I have not tried to test this against the backported asyncio library for Python 3.3 but it should work fine since new syntax is not used anywhere in the library, only in tests.
• A conftest.py was added to skip tests involving the async/await behavior on Python versions that do not support it.
• Promise.promise_for_dict was added to the documentation, with a description directly copied from its docstring.
• The documentation has been changed to indicate that the class method for creating a failed Promise is called Promise.rejected rather than Promise.reject -- the latter is an instance method.

[coveralls]

Coverage decreased (-0.7%) to 93.496% when pulling 7b52b043ba2962130b3974e559f0e3464fb4e1ec on csboling:master into 127a202f553f073e5c891231c8227c6800d4c210 on syrusakbary:master.

[coveralls]

Coverage decreased (-0.7%) to 93.496% when pulling ca0387b7065c5d4ece563f948122ab8670f36176 on csboling:master into 127a202f553f073e5c891231c8227c6800d4c210 on syrusakbary:master.

[csboling]

The 2.7 build is failing because testing the async/await code relies on the pytest-asyncio plugin, which doesn't work in 2.7. The build passes for 2.7 with Tox on my local machine, but I'm not sure how to get Travis to install a testing dependency only for some versions.