Add 'REQUIRE' clause to the grant command. (Issue #371)

Added the option add 'REQUIRE' clause to the grant command. This allow SSL based authentication.
http://dev.mysql.com/doc/refman/5.5/en/grant.html

Example:
```puppet
mysql_grant { 'user1@host1/*.*":
  ensure => 'present',
  options => ['GRANT'],
  privileges => ['ALL'],
  table => '*.*',
  user => 'user1@host1',
  grant_require => ['SUBJECT "/CN=user1/OU=Company"', 'ISSUER "CompanyCA"' ] ,
}
```
generates:
```
GRANT ALL PRIVILEGES ON *.* TO 'user1'@'host1' REQUIRE ISSUER 'CompanyCA' SUBJECT '/CN=user1/OU=Company' WITH GRANT OPTION
```