Add type check to travis

[c24t]

Piling onto #30, this PR adds type checks via mypy to travis for the API package.

These are pretty strict defaults; I followed @Oberon00's approach in #24 and enabled every available option that wasn't already incompatible with our code. I had to omit --disallow-any-expr because of contextmanager, but there may still be a way around this.

See https://mypy.readthedocs.io/en/latest/config_file.html for details on these options.

[Oberon00]

A potential way around this that I can see is using typing.ContextManager[Span] instead of @contextmanager and Iterator[Span]. typing.ContextManager is only available since Python 3.5.4 though and I think we would need more boilerplate code. The 3.5.4 part could probably be worked around with some if MYPY which we should consider anyway, since typing is new in 3.5. Or we use https://pypi.org/project/typing/ and typing.IS_TYPE_CHECKING (3.5.2)

[Oberon00]

Looks mostly straightforward. The only thing that irks me is the #type: ignore in the __init__.pys. I wonder why mypy has problems with __path__. A mypy bug?

[reyang]

LGTM

[c24t]

typing.ContextManager is only available since Python 3.5.4 though and I think we would need more boilerplate code

I don't know that it's worth it to add the extra boilerplate, and it looks like mypy correctly checks calls to e.g. start_span even though the context manager itself isn't typed. I'll leave it as-is and we can revisit if this causes problems down the road.

The 3.5.4 part could probably be worked around with some if MYPY which we should consider anyway, since typing is new in 3.5

It sounds like we've got two options: either install the typing backport if we're running on 3.4 or eat the error while importing typing and change references to the module in type annotations to strings. So from:

    def start_span(self, name: str, parent: 'Span') -> typing.Iterator[Span]:

to:

    def start_span(self, name: str, parent: 'Span') -> 'typing.Iterator[Span]':

The second option seems like overkill and doesn't let us use e.g. typing.ContextManager, but requiring a third party library for optional type checks doesn't sound any better.

[c24t]

The only thing that irks me is the #type: ignore in the __init__.pys. I wonder why mypy has problems with __path__. A mypy bug?

Luckily this is fixed -- for now -- because of the package move. It does look like this is an open bug in mypy: python/mypy#1422.

[c24t]

I added typing as a requirement for pre-3.5 python versions in 15b30bc.