Refactor request creation to use list of NameValuePairs
Fix handling of WithdrawalLimitExceptions

Minor typo which results in an Exception being thrown
when handle is called in WithdrawalLimitExceptionHandler
Convert NeopetsURL to a class from enum

To handle variable URLs, a class provides a mechanism
to instantiate new URL objects.

This may be later reverted if this cabability is
no longer needed.
Remove support for String urls in NeopetsRequest

URLs will only be passed into NeopetsRequest constuctor
as NeopetsURL enum.
Refactor NeopetsRequest construction with chaining of setters
Abstract NeopetsRequest construction to mashallers in NeopetsBankClient
Refactor passing of parameters to request construction

Add option to pass ParametersBuilder into NeopetsRequest
constructor to simplify syntax further