Merge pull request #104 from swanandp/add_rake_console_task

Added convenience Rake task to open a console with the gem pre-loaded.
Added support for:

1. Estimate Model - with example attributes (TODO: Support all attributes)
2. Estimates Service - only retrieve is tested right now (TODO: Test & support all)

Next up: Separate commit for Specs
Added billing_email support
Id, SyncToken & MetaData are not part of the header.
Estimates support in Windows service
attr name must be uniform
Corrected field name for 'Estimate#doc_number', it is referred as doc_number everywhere else and not document_number
Merge branch 'add_estimates_support'

* add_estimates_support:
  Corrected field name for 'Estimate#doc_number', it is referred as doc_number everywhere else and not document_number
  attr name must be uniform
  Estimates support in Windows service
  Id, SyncToken & MetaData are not part of the header.
  Added billing_email support
  Added support for: