Feature matrix & conditioned doc generation

- Uses `make` because currently `cargo` fails if invoked as `--features=""`, hope this will be fixed soon

- Tests that library builds without any features and with `tlsv1_1`, `tlsv1_2` features enabled. Note, that `sslv2` isn’t in test case because on modern linux (like on Travis servers) `SSLv2_method` isn’t in `libssl.so` so it fails on link stage.

- Generates and uploads docs only in no features build

- Uses `cargo doc` so docs include also mentions of all available features (although doesn't work currently as it seems that `rustdoc` ignores feature-gated items.