DynamoDB On-Demand Support

[fishcharlie]

Summary:

This PR adds an option for ON_DEMAND for throughput for your schema to create the table using PAY_PER_REQUEST Billing Mode.

Code sample:

Schema

var User = new dynamoose.Schema({
  id: Number,
  name: String
}, {
  throughput: "ON_DEMAND"
});

GitHub linked issue:

Closes #470

Other information:

• Currently DynamoDB Local doesn't support the BillingMode property. Once support for that is added, we need to update this code. I have added comments for changes needed once that support is added.

Type (select 1):

• Bug fix
• Feature implementation
• Documentation improvement
• Testing improvement
• Test added to report bug (GitHub issue #--- @---)
• Something not listed here

Is this a breaking change? (select 1):

• 🚨 YES 🚨
• No
• I'm not sure

Is this ready to be merged into Dynamoose? (select 1):

• Yes
• No

Are all the tests currently passing on this PR? (select 1):

• Yes
• No

Other:

• I have searched through the GitHub pull requests to ensure this PR has not already been submitted
• I have updated the Dynamoose documentation (if required) given the changes I made
• I have added/updated the Dynamoose test cases (if required) given the changes I made
• I have run npm test from the root of the project directory to ensure all tests continue to pass
• I agree that all changes made in this pull request may be distributed and are made available in accordance with the Dynamoose license
• All of my commits and commit messages are detailed, explain what changes were made, and are easy to follow and understand
• I have confirmed that all my code changes are indented properly using 2 spaces
• I have filled out all fields above

[coveralls]

Pull Request Test Coverage Report for Build 729

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 85.192%

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Totals
Change from base Build 726:	0.2%
Covered Lines:	1887
Relevant Lines:	2146

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💛 - Coveralls