New option to auto generate sql files

Add the option `--sql-file` that can be passed with the `db-migrate` command or set by the `database.json` config file.
I don't know if someone else is interested in this feature, but in my job we prefer to set all the up and down statements on a .sql file.

So, when creating a new migration as:
`db-migrate create migration --sql-file`

These two sql files are also created:
`migrations/sqls/20140323230607-migration-up.sql`
`migrations/sqls/20140323230607-migration-down.sql`

And the file `migrations/20140323230607-migration.js`, instead of having the empty `exports.up` and `exports.down`, has the auto-generated code that load the sql files:
````
var dbm = require('db-migrate');
var type = dbm.dataType;
var fs = require('fs');
var path = require('path');

exports.up = function(db, callback) {
  var filePath = path.join(__dirname + '/sqls/20140323230607-migration-up.sql');
  fs.readFile(filePath, {encoding: 'utf-8'}, function(err,data){
    if (err) return console.log(err);
      console.log('received data: ' + data);

    db.runSql(data, function(err) {
      if (err) return console.log(err);
      callback();
    });
  });
};

exports.down = function(db, callback) {
  var filePath = path.join(__dirname + '/sqls/20140323230607-migration-down.sql');
  fs.readFile(filePath, {encoding: 'utf-8'}, function(err,data){
    if (err) return console.log(err);
      console.log('received data: ' + data);

    db.runSql(data, function(err) {
      if (err) return console.log(err);
      callback();
    });
  });
};
````

I added some unit tests for the changes I made on migrations.js and integration tests for the use of this new option.