Feature matrix & conditioned doc generation

- Ensures that library builds and passes all tests 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
  builds fine but fails on linking tests.

- For simplification it generates docs after every build, but 
  uploads them once - if it is `master` branch and not a pull
  request and DOC_FEATURES is set to list of all available 
  features