Add client certificate support

[seanmonstar]

Closes #43
Closes #176

Maybe? Is Identity too generic of a type name?

[seanmonstar]

My only hesitation is in the naming. Looking around at other libraries, I don't see any others calling this an Identity. Usually it's named something about "cert". In some cases, it's overloaded, such that cert really means cert + key + password, and in others, cert, key, and password options are set individually.

I kind of feel like neither of the best. A client certificate is more than just the cert, but it needs all 3, not optionally some of them. The latter concern is covered by this being a single type, like Identity. It would seem weird to call this a Certificate, since it's actually more than that. Perhaps Identity is the best?