Add on Demand mode

[aneilbaboo]

This adds a new schema attribute, billingMode, which can be set to 'PROVISIONED' (the default) or 'PAY_PER_REQUEST', which causes the table to run in On Demand mode. This value can also be provided on a per-table basis to the dynogels.createTables` function.

• Set billingMode as an attribute of the schema:

const Account = dynogels.define('Account', {
  hashKey : 'id',
  schema : {
    id : Joi.string()
  },
  billingMode: 'PAY_PER_REQUEST'
});

• Pass it billingMode as a parameter to dynogels.createTables (overrides schema):

dynogels.createTables({
  'Account': {
    billingMode: 'PAY_PER_REQUEST'
  }
}, function(err) {
  if (err) {
    console.log('Error creating tables: ', err);
  } else {
    console.log('Tables has been created');
  }
});

Fixes #182

[aneilbaboo]

Rebased on current master and addressed the comment from @rchl.