Allow bypassing of mimetype and encoding guessing

[moggers87]

Following on from #18

sendfile() currently checks if the requested file exists regardless of chosen backend. However, most backends actually support serving files that are not necessarily accessibly to the Django app.

• File exists checks should only happen when data for headers is not available (implying the user wants us to do it for them)
• Add a kwarg for content length
• Simple and development backends assume that the file exists if they are called, either make those backends aware or wrap the backend call in an appropriate try/except