Rescue MultiJson Parser Error

Hi,

I notice that this [example](https://github.com/aaronlahey/finances_ruby_client/blob/bd8bd52c03671dbd99a7fe4ebfec5b9a8e630d0e/spec/finances/merchants_spec.rb#L68) will fail because a `MultiJson::ParserError` is raised before an `ValidationError`. (An empty response body is an invalid JSON object.) 

You might want to consider either:
1. Rescuing it here like I did, or
2. Rescuing it in the [`Finances::Merchants.create` method](https://github.com/aaronlahey/finances_ruby_client/blob/bd8bd52c03671dbd99a7fe4ebfec5b9a8e630d0e/lib/finances/merchants.rb#L32)

And captures and returns the error messages and backtrace.