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
Moving keystore definition into settings
Use the system keystore by default

Default system keystore is in:
$JAVA_HOME/lib/security/jssecacerts
or in:
$JAVA_HOME/lib/security/cacerts

Custom keystore can be set either in /etc/sysconfig/gitbucket by
specifying the following option:
GITBUCKET_JVM_OPTS="-Djavax.net.ssl.trustStore=/path/to/your/cacerts"
or in Gitbucket's System Settings.
Merge pull request #190 from jtyr/master

Adding LDAP StartTLS support