add delete to bill service
Merge pull request #84 from pavelpachkovskij/master

add delete to bill service
Attributes total_tax within the TransactionTaxDetail model and rate_percent
within the SalesItemLineDetail model should not be set by default. Using
the :to_xml with no nil? check automatically creates that attribute within
a request. Using the proc returned from to_xml_big_decimal method does
do a nil check.

Lastly, the percent_based attribute
within the TaxLineDetail model should be a ROXML boolean.
Merge branch 'dont-send-if-nil' of github.com:minimul/quickbooks-ruby into dont-send-if-nil