Include _api.c and _api.h.
Add a way to specify imagemagick configuration.

Specifically, the SANPERA_IMAGEMAGICK_{C,LD}FLAGS environment variables can be
filled in to specify what flags to use.
Write out cffi configuration to a JSON file.

This has several advantages. First, pkg-config won't have to be called on every
sanpera import. Second, built binaries will be more portable across systems.
cffi uses the CFLAGS/LDFLAGS as part of the input into its hashed module names,
so if a binary is shipped to a machine that doesn't have knowledge of what the
CFLAGS and LDFLAGS were, it won't have to guess (and therefore not find the
module).
Update gitignore.
Add CoalesceImages to the API.