Added documentation for invoice creation

Added documentation for invoice creation which shows how a customer can be linked to the invoice without making an extra round trip to xero.
Revert indentation changes.
Merge pull request #71 from fimmtiu/log-http-traffic

Allow Logging of Raw HTTP Traffic
Merge pull request #51 from minhajuddin/master

Added documentation for invoice creation without extra roundtrip
Correct bogus xpath specification for ValidationErrors.
Merge pull request #74 from fimmtiu/fix-validation-errors-xpath

Correct Bogus XPath Specification for ValidationErrors.
Add error messages to records that batch_save can't sync.
Merge pull request #75 from fimmtiu/add-errors-to-records

Add Error Messages to Records That batch_save Can't Sync
Merge branch 'master' of https://github.com/waynerobinson/xeroizer into develop

* 'master' of https://github.com/waynerobinson/xeroizer:
  Add error messages to records that batch_save can't sync.
  Correct bogus xpath specification for ValidationErrors.
  Revert indentation changes.
  Added documentation for invoice creation