Add additional fields for invoices

- Shipping address (SADDR1 to SADDR5)
- Sales rep (REP)
- Invoice title and memo (INVTITLE and INVMEMO)
- Purchase order number (PONUM)
Merge pull request #6 from jphpsf/add-invoice-missing-fields

Add additional fields for invoices
Add Contributors to README.
Update CHANGELOG.md
Bump to 0.3.0