Added gdImageFile(),  gdImageCreateFromFile() and gdSupportsFileType().

These are convenience functions which load or save image data to a
file.  They are roughly equivalent to opening a file handle with
fopen() and calling gdImageCreateFrom*() or gdImage*() on the FILE
pointer.  However, these functions identify the input or output format
from the filename suffix and call the appropriate read or write
function accordingly.

gdSupportsFileType() can be used to test if a specific file format
is supported.

Most scripting interfaces already do something like this but now
there's support for doing it from C as well.

This change also adds test cases for the code and naturaldocs
documentation.