User-provided data in verify
Simpler setter for verify with data

There is no need in wrapping function in option as there is no
sense in providing data without function.
Add a dummy bn_is_zero C dependency to wrap BN_is_zero

This is necessary because on some architectures BN_is_zero
is a CPP macro, so trying to link against it in an `extern "C"`
block causes a linker error.

This also introduces a build command to Cargo to compile
the bn_is_zero wrapper.
Merge pull request #58 from vhbit/verify-data

User-provided data in verify
Merge pull request #64 from reem/fix/big-num-is-zero

Add a dummy bn_is_zero C dependency to wrap BN_is_zero
Changes made to support 0.12.0-dev ( October 2, 2014 )
Clean up warnings
Make errors human readable

Change error messages from numeric codes to human readable strings. This makes debugging failures much easier.
Address CR comments and add a test
Fix error messages
Load crypto error strings
Merge pull request #59 from jroesch/better-errors

Make errors human readable
Ignore error string text

cc #65
Ignore stderr from openssl s_server
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.