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>
```
Corrected typo in exception name: 'InvaidPermissionError'.
Made the change backwards-compatible & added a spec.

We don't want to hose anyone who's already catching 'InvaidPermissionError', after all.
Made the spec more elegant.
Fix updated_date_utc being parsed in local timezone, fixes #43
Fix HTTP 503 issues
Merge pull request #62 from latentflip/fix-43

Fix updated_date_utc being parsed in local timezone, fixes #43
Merge pull request #63 from latentflip/fix-503

Fix HTTP 503 issues
Added status to bank transaction
Bank transaction list only contains summary
Merge pull request #53 from zmoazeni/master

Enables TaxRate Status Field
Merge pull request #59 from fimmtiu/fix-typo-in-exception-name

Corrected Typo in Exception Name InvaidPermissionError
Update RDOC Rakefile Include
Merge pull request #65 from latentflip/bank-transactions-list-summary

Bank Transaction List Only Contains Summary
Merge branch 'bank-transaction-status' of https://github.com/latentflip/xeroizer into latentflip-bank-transaction-status
Merge pull request #66 from latentflip/bank-transaction-status
Update Bank Transaction Status Descriptions
Merge branch 'master' of github.com:waynerobinson/xeroizer
Fix ordering
Merge pull request #67 from latentflip/fix-ordering

Fix ordering
Heh. Whoops.