Better error handling in cert generation

Now it should correctly free all resources in case
of failure.
Merge pull request #60 from vhbit/cert-gen-cleanup

Better error handling in cert generation
Potential fix for #68
Merge pull request #69 from vhbit/bn-zero-fix

Fixes #68: error on linking `bn_is_zero`
Feature matrix & conditioned doc generation

- 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.