Support for reading and writing PNG metadata

[joannajw]

This pull request adds the feature requested in issue #164. It adds support for writing and reading metadata in PNG files. setMetadata() sets a PNG tEXt chunk with the key lwip_data and comment as the input string. getMetadata() returns the contents of the first tEXt chunk found with the key lwip_data, or null if none exists.

Setting metadata:

lwip.create(width, height, function(err, img) {
    img.setMetadata("test");
    image.writeFile("test.png", function(writeErr) {});
});

Getting metadata:

lwip.open("test.png", function(err, img) {
    var metadata = img.getMetadata();
    console.log(metadata);
})

[EyalAr]

How are different types of 'data' handled?

Eventually this value will go all the way to here, at which time it will be casted to a string.

If users pass here an object, for example, do they expect us to serialize it for them? What if they pass in a function?

In the docs we say that 'data' should be a string. So we should probably enforce it here by doing type checks and throwing an exception if needed, as done in other methods.

[joannajw]

Good point, I think it would make sense to only allow metadata to be set to a string, so I added that check.

I've also added the option to pass in null to setMetadata, which will cause the image to be saved without any tEXt chunks when writeFile is called.

[EyalAr]

@joannajw It looks great. Please see the comments I've added.

Thanks!

P.S. please add the tests/.DS_Store to .gitignore.

[joannajw]

Thanks for the comments! I've made the changes you mentioned and removed unnecessary comments. Let me know what you think.

[EyalAr]

@joannajw
Merged. Thanks for the great PR.

I'll release a new version of lwip with your changes shortly.

[joannajw]

Thanks so much for the merge!

I was wondering if you have an idea of when a new release will be made, so we can update our project to incorporate the new feature as well?

[EyalAr]

@joannajw I'll try to push it this weekend.

[joannajw]

Awesome, thanks!

[darongleng]

the comment should this:
// TODO: implement getting metadata from JPEGs; this is a placeholder