Enables TaxRate status field

The reference manual doesn't mention it
(http://blog.xero.com/developer/api/Tax%20Rates/) but the API previewer
tells you whether the TaxRate is ACTIVE or DELETED.

```
<TaxRate>
  <Name>My Rate</Name>
  <TaxType>TAX003</TaxType>
  <CanApplyToAssets>true</CanApplyToAssets>
  <CanApplyToEquity>true</CanApplyToEquity>
  <CanApplyToExpenses>true</CanApplyToExpenses>
  <CanApplyToLiabilities>true</CanApplyToLiabilities>
  <CanApplyToRevenue>true</CanApplyToRevenue>
  <DisplayTaxRate>3.2000</DisplayTaxRate>
  <EffectiveRate>3.2000</EffectiveRate>
  <Status>DELETED</Status>
</TaxRate>
```