Add basic support for keys

[linas]

Add minimal support for keys; this is needed for IPNS support.

[linas]

BTW, You may dislike the API I added: I wanted to have optional parameters with default values, and for that, I had to place them last, after the return value. This is obviously ugly. Another alternative would be to pass in a json holding everything as a single input, but this also seems .. pointlessly ugly. Beats me, Suggest something better, I'll code it up.

[vasild]

Hi,

I removed the optional-ness from key_type and key_size and also changed the type of the latter from std::string to size_t. See commits 80b1fb7 and 87a6cce.

Thanks!