Initial release

Features:

• Class methods to read de current tenant, and set the current tenant for a piece of code.
• A class to handle multitenant resources without having to deal with the current tenant on your business logic.
• A Rack middleware to choose the request tenant on rack applications with custom or built-in strategies.
  • Built-in strategies:
    • Header: Read the tenant from a request header.
    • Host: Choose the tenant depending on the request host.
    • Default: Return always the same tenant.