Feature matrix & conditioned doc generation

- Tests that library builds with `sslv2` and `tlsv1_2` features enabled (`tlsv1_1` should be the same with 
`tlsv1_2`)

- Generates and uploads docs only in default build

- Uses `cargo doc` so docs include also mentions of all available features.

FIXME: check if `rustdoc` actually makes it clear that feature is available only with specific feature
enabled