Methods for QuickBooks entity operations

[mockingmoon]

I would like to contribute to this project to make it a fully-equipped Node.js client for Intuit.
I have recently worked with an Intuit QuickBooks App development for Node.js wherein I used intuit-oauth and wrote my own methods for different operations on QuickBooks. Can we integrate Intuit operations into this package? Thus everything can be found at single place and it will be Intuit's own package.
This fork currently only performs create and get operations for the most commonly used QuickBooks online entities. I am planning to work on it further to add all QuickBooks online operations and additional features to this package.
Please add me as a contributor if my proposal (and code) is acceptable to you.

I assure the following -

1. The code written by me will follow the required standards (and I dare say the style will be almost indistinguishable from the original code, except for some variable names).
2. All my additions are tried and tested, and will not break any existing code.

Additional explanation -
I have added realmId to OAuthClient object. This will allow a person to directly enter realmId while creating the client instance. Thus if a person has the refresh token and realmId for an app, they can directly set the realmId and refresh their access token without having to authenticate their app again.

[abisalehalliprasan]

Thanks, @mockingmoon . I will take a look at the PR and get back to you. Appreciate your help.