Fix up some cffi stuff

This is several cffi-related changes. 

1. Updated the gitignore.
2. Bundled `_api.[ch]` both in the sdist and in the installed package.
3. Added `SANPERA_IMAGEMAGICK_{C,LD}FLAGS` for specifying how to compile ImageMagick in places without `pkg-config`.
4. Moved the cffi-built extension module into the `sanpera` package instead of having it be top-level.
5. Stopped trying to query for `CFLAGS` and `LDFLAGS` at runtime. See 279276ff26572b43189b700e7335a588fb2d0514 for more details; this one is definitely the most involved. It solves problems related to trying to build binaries which are distributed to machines without development headers.
6. Added `CoalesceImages` to the API, which was forgotten.