Improve Fiona examples.

They are all excellent and don't need huge improvement. But I
did the following:

- Changed collection() to open().
- Removed 'r' mode args since this is the default.
- Switched to single quotes.
- Avoid unnecessary schema copying (saves a line of code!)
- Removed duplicate code in the (awesome!) pyproj example.

I ran the modified examples to confirm they work. The stereographic
example is really good.