Adding LDAP StartTLS support

Some LDAP server do not allow authenticate with unencrypted password. This patch is adding the StartTLS support which takes care of the encryption.

In order to enable the StartTLS, go to "System Settings" and select the "Enable StartTLS" in the Authentication section. Then make sure that you add your LDAP certificate into the Java keystore:

$ keytool -import \
          -file /etc/pki/tls/certs/cacert.pem \
          -alias myName \
          -keystore /var/lib/gitbucket/keystore

You can list all keys from the keystore like this:

$ keytool -list -keystore /var/lib/gitbucket/keystore