Support for rfc3062 Password Modify, closes #163

This implements the password modify extended request
http://tools.ietf.org/html/rfc3062

Change existing password:

```ruby
dn = 'uid=modify-password-user1,ou=People,dc=rubyldap,dc=com'

auth = {
  method: :simple,
  username: dn,
  password: 'passworD1'
}

ldap.password_modify(
  dn: dn,
  auth: auth,
  old_password: 'passworD1',
  new_password: 'passworD2'
)
```

Or get the LDAP server to generate a password for you:

```ruby
dn = 'uid=modify-password-user1,ou=People,dc=rubyldap,dc=com'

auth = {
  method: :simple,
  username: dn,
  password: 'passworD1'
}

ldap.password_modify(
  dn: dn,
  auth: auth,
  old_password: 'passworD1'
)

ldap.get_gen_passwd_result #=> 'VtcgGf/G'
```