Ability to pass custom fields to payment request

I've implemented the ability to pass custom fields to instant payment requests.  Initially, I created a branch to implement the `NOSHIPPING` option, but after reading the API documentation (https://www.x.com/sites/default/files/pp_nvpapi_developerguide.pdf, warning, it's huge) I noticed there were about 10 fields I wanted to use but weren't implemented yet.

Having the ability to pass custom fields adhoc to the request would be more error prone, but also means this gem can support the whole API (including updates) without having to manually implement every field.

Added a `:custom_fields` hash to the `Paypal::Payment::Request` attributes.  Implemented with this is replacement of lower case `n` in field names with the current index (lower case `n` is the standard for indexed values in the doc).  Updated spec to test custom fields.

If you don't think this fits, please close the pull request, I'll be able to keep using this for my own stuff by referencing my repo.

Mick