fixes

wip

add new helper methods.

add invoiceFor method

Merge remote-tracking branch 'origin/6.0' into 6.0

Merge pull request laravel#245 from mikepostma/mikepostma-patch-incre…

…mentAndInvoice

Fix incrementAndInvoice function

fix some starting balance issues.

Merge remote-tracking branch 'origin/6.0' into 6.0

remove php 5.4

allow 2.7