Small code cleanup.

Removed non-exported prototypes from gd.h.  Where possible, made the
functions static and moved the rest of the prototypes to gd_intern.h.

Also removed redundant macros MIN/MAX macros from gd_interpolation.c
(since they're also defined in gd_intern.h) and tidied the formatting
a little.
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.
Added gdnametest and gdCopyBlurred to .gitignore.
Merge branch 'master' of https://bitbucket.org/libgd/gd-libgd
Added function 'gdImageGetInterpolationMethod()'.

This retrieves the interpolation method set via
gdImageSetInterpolationMethod().  It is a function instead of a macro
for the benefit of VMs that need a function pointer to call.
Started importing the old manual into Naturaldocs comments.

This changeset imports documention for gdImage to gdImagePng*() into
Natural Docs.  Documention is based on the text of the original
manual, version 2.0.36 but adapted to better suit the format.
Subsequent changesets will introduce docs for subsequent manual
entries.

naturaldocs, when present, is invoked by bootstrap.sh.  The completed
manual will be in docs/naturaldocs/html/index.html.  It can also be
explicitly invoked by running docs/naturaldocs/run_docs.sh.

This change also removed docs/naturaldocs/project/Menu.txt, since it
currently contains no non-generated content and is prone to introduce
noise into the changeset.
Merge branch 'master' of https://bitbucket.org/libgd/gd-libgd

Added gdGetInterpolationMethod; added naturaldocs from the manual.