Create a Boku API client in Solitude (bug 992376)

- Create an MD5 signer for request signatures
- Parse XML content returned
- Create wrapper methods for interacting with commonly used endpoints
    - Get pricing tiers
    - Start a transaction
    - Check the status of a transaction
Merge pull request #210 from JaredKerim-Mozilla/992376

Create a Boku API client in Solitude (bug 992376)