New api call: getImageMeta()

[dberesford]

Returns the following image properties (which are not tags but still very handy to have when processing images):

{ fileName: '/path/books.jpg',
  fileSize: '993738',
  mimeType: 'image/jpeg',
  pixelHeight: '1200',
  pixelWidth: '1600' }

Exiv2 also outputs these by default, along with the high level summary exif tags.

[drewish]

This needs to be reworked to use NAN.

I'd suggest skipping the filename since that's passed in. I've also been thinking about how we could pass in the file contents as a buffer or filename. In the case of reading from a buffer there would be no filename. The other properties seem like they'd work though.

[whitelynx]

@drewish It looks like @bertrandom implemented reading/writing using Buffer at https://github.com/bertrandom/exiv2node-buffers, but it's definitely quite out of date. (it at least won't compile on Node.js v5.3.0)

@dberesford I'd love to see this PR updated to use NAN; I've also wanted to get this info along with the standard metadata tags.