Make the class public and add a parameterless constructor

Hi James - great work on this project really like what you are doing: hope this helps;

Adding a parameterless constructor resolve issue in TableUserStore.cs FindAsync which previously resulted in the following error:

<ExceptionMessage>
TableQuery Generic Type must provide a default parameterless constructor.
</ExceptionMessage>

Regards,

James.
Merge pull request #6 from jameskeongchen/patch-1

Make the class public and add a parameterless constructor