Add filtering

[jtl3d]

Hey, this is great stuff! I'm in the middle of building an app and I'm definitely going to utilize this. I am most concerned with retrieving transactions, and I am going to need some filtering capabilities. So I'm creating this issue in case you find it useful, but I will likely end up creating a pull request if there is no movement on your end on this.

I'm thinking something like this:

CJ.getTransactions({
   where: {
      postingDate: 'event',
      startDate: ...
   }
});

What do you think?