support new TCPConnector param fingerprint

[requiredfield]

Fixes #350

[asvetlov]

Please replace '...' with example of arbitrary fingerprint. It may help to newcomers very well.

[requiredfield]

@asvetlov Thanks, implemented all your suggestions. Please let me know if latest patch looks better.

Just wasn't sure what you wanted here. Yes, the property returns a string (or None if not set). Happy to rename this (something like expected_fingerprint?) everywhere rather than verify_fingerprint if you prefer?

[requiredfield]

pushed a new version with verify_fingerprint renamed to expect_fingerprint, as well as improvements to weirdness cargo-culted from the urllib3 code

[asvetlov]

Should we have a way to pass bytes fingerprint?

[requiredfield]

Should we have a way to pass bytes fingerprint?

Something like 8661202?

[asvetlov]

Sorry for bad guess, req.ssl may clash with ProxyConnector.

Please use transport.get_extra_info('sslcontext') instead as flag that transport.get_extra_info('socket') has certificate info.

[asvetlov]

Sorry for long review, I'm striving to make code perfect.
Thanks you for patiently fixing my comments.

After looking on the last version I incline to accept bytes-only fingerprint as connector parameter.
I guess to rename it to just fingerprint -- short name is better.

We need also for str hexlified fingerprints -- both with : inside string and without it.
I guess to add function for converting str fingerprints to bytes ones. aiohttp/helpers.py is good place for that function.

Library users can find the helper function easy if you mention it in TCPConnector documentation with cross-reference.

Clean separation to byte-ish fingerprint for usage in connector and helper for converting user-readable str to bytes required by connector makes sense I feel.

FingerprintMismatch should operates with bytes for got and expected, they are have good enough repr(). We can unhexlify those sometimes in future if we'll find the repr is not readable well.

Both fingerprint property and ctor parameter should be documented in docs/client_reference.rst with ..versionadded: 0.16 tag.

We are in the middle on getting rid of sphinx autodoc generated documentation (docstrings in connector.py still not converted to plain text for example), you may add autodoc docstring to helper function.

If you are too boring in writing documentation please left it to me -- I'll accept your PR after code and tests done but very appreciate your help in documentation also.