Fix spelling, update tutorial link, and add info about changing logger targets
Merge pull request #40 from minimul/readme-update

Fix spelling, update tutorial link, and add info about changing logger targets.
Kept receving a:
```
Fetching source index from http://rubygems.org/
Resolving dependencies...
Bundler could not find compatible versions for gem "nokogiri":
  In Gemfile:
    quickeebooks (>= 0) ruby depends on
      nokogiri (~> 1.5.9) ruby

    nokogiri (1.6.1)
```
.. when trying to bundle in a client project
after updating quickbooks-ruby to the latest. Removing the
pessimistic constraint allowed the bundle to go thru fine.

Let's roll back this change as many apps are going to be running
quickeebooks also.
Merge pull request #41 from minimul/nokogiri

Kept receving a:
Include base_service after service_crud
Include ServiceCrud in BaseService
Remove include ServiceCrud

Its already included in BaseService class
Merge pull request #42 from whereisciao/include-crud-in-base-service

Include crud in base service
SalesReceipt.ship_method_ref is a simple String and not a BaseReference. Closes #45
Dont set Item#rate_percent under the hood

This can cause unexpected results. For example: the Item#unit_price
would not persist in quickbooks as expected because the gem would send a
request with RatePercent: 0.0. That would affect the unit_price setting
it to nothing (0)

Not 100% sure on this but I believe all BigDecimal attributes could take
advantage of this to_xml_big_decimal method introduced here
Use to_xml_big_decimal for other Item attributes
Merge pull request #46 from huoxito/item-unit-price

Dont set Item#rate_percent under the hood
Add time activity service and model.