Fix #24. Moves Use Case and Custom Models to before Iris Stack.

Also fixes the title, removes some bullet points, fixes up intro to include Sherpa, Specview and NED references.

The Use Case is new. It's more general, and follows more of a typical Iris session rather than a completely specific use case with steps 1, 2, and 3. It still follows the steps of the PKS 1127-14 use case, but it's a lot more general.

You'll notice a lot of commented-out stuff in the use case section. The single-comments (preceded by a single '%') are to use if you don't like the current use case section. You can just uncomment these, and they'll make a brief version of the old Use case (where the user does specific tasks to get her SED build). The double-% comments ('%%') should not be used... they're just there if I want to go back to something else... Let me know if you want me to clean it up :)