Add Accounts

Fix headers
Merge pull request #14 from dongilbert/feature-accounts

Add Accounts
Start tokens implementation.
Add DS_Store to gitignore
Merge remote-tracking branch 'upstream/master' into feature-tokens
More tokens implementation. Still thinking about how to do the create.
Fixed typos
Merge pull request #15 from pborreli/typos

Fixed typos
Finish InvoicesTest
Finalize tokens implementation w/tests. Add constants to StripeTestCase.
Merge pull request #16 from dongilbert/feature-tokens

Tokens