Add :marketplace rspec metadata helper

Tagging an example group with the `:marketplace` metadata key will
run a `before(:each)` hook that calls `make_marketplace`. Most examples
require a marketplace to exist, so this should help DRY up a lot of boilerplate
setup code
default the Balanced::StandardError#initialize param to nil
Balanced::StandardError super's to Balanced::Error

Balanced::Error then `super`'s to `::StandardError` which fills out
the `to_s` and `inspect` methods nicely to the error message. This
makes it much easier to decipher the nature of the error from the
command prompt, or in various exception handling libraries.
add Balanced::UnassociatedCardError exception class
raise a Balanced::UnassociatedCard error when charging an unassociated card
better inline docs
support account, account_uri, customer and customer_uri
spec ensuring that associated cards can be debited
Merge pull request #98 from ajsharp/raise-error-when-debiting-unassociated-cards

Raise an exception when trying to charge an unassociated card