Correctly report TLSv1.2 only ciphers as negotiable with TLSv1.2

Previously scan would report:
prio  ciphersuite                  protocols                    pfs_keysize
1     ECDHE-RSA-AES128-GCM-SHA256  SSLv3,TLSv1,TLSv1.1,TLSv1.2  ECDH,P-256,256bits
2     ECDHE-RSA-RC4-SHA            SSLv3,TLSv1,TLSv1.1,TLSv1.2  ECDH,P-256,256bits

Now it correctly reports:
prio  ciphersuite                  protocols                    pfs_keysize
1     ECDHE-RSA-AES128-GCM-SHA256  TLSv1.2                      ECDH,P-256,256bits
2     ECDHE-RSA-RC4-SHA            SSLv3,TLSv1,TLSv1.1,TLSv1.2  ECDH,P-256,256bits
update examples from README

since now the scan reports protocols correctly, update the example
to illustrate that
report key size used in server's certificate

Extend the report to show also server certificate key size:
prio  ciphersuite              protocols                    pubkey_size  pfs_keysize
1     ECDHE-RSA-AES128-SHA256  TLSv1.2                      2048         ECDH,P-256,256bits
2     ECDHE-ECDSA-AES128-SHA   SSLv3,TLSv1,TLSv1.1,TLSv1.2  256          ECDH,P-256,256bits
3     AES128-SHA               SSLv3,TLSv1,TLSv1.1,TLSv1.2  2048
4     RC4-MD5                  SSLv3,TLSv1,TLSv1.1,TLSv1.2  2048
5     EXP-RC4-MD5              SSLv3,TLSv1,TLSv1.1,TLSv1.2  2048         RSA,512bits
Report the signature type used on server certificate

Parse the certificate used by server and report the signature used:

prio  ciphersuite              protocols                    pubkey_size  signature_algorithm    pfs_keysize
1     ECDHE-RSA-AES128-SHA256  TLSv1.2                      2048         sha1WithRSAEncryption  ECDH,P-256,256bits
2     ECDHE-ECDSA-AES128-SHA   SSLv3,TLSv1,TLSv1.1,TLSv1.2  256          ecdsa-with-SHA512      ECDH,P-256,256bits
3     AES128-SHA               SSLv3,TLSv1,TLSv1.1,TLSv1.2  2048         sha1WithRSAEncryption
4     AECDH-RC4-SHA            SSLv3,TLSv1,TLSv1.1,TLSv1.2  0            None                   ECDH,P-256,256bits
5     RC4-MD5                  SSLv3,TLSv1,TLSv1.1,TLSv1.2  2048         sha1WithRSAEncryption
6     EXP-RC4-MD5              SSLv3,TLSv1,TLSv1.1,TLSv1.2  2048         sha1WithRSAEncryption  RSA,512bits
check if certificate used by server is trused

Use system trust anchors to check if certificate chain used by server
is actually valid.
update README with new examples

New features = new examples
make default output more narrow

If server uses the same certificate for all connections, it's
useless to print the same information over and over.

In such case, omit those columns and print a summary at the end
Merge pull request #12 from tomato42/certificate-scanning-02

Certificate scanning 02 (alternative version)