Make handlers return buffers

[eduardoboucas]

This PR changes the various handlers so that they return a buffer instead of a stream. This avoids multiple conversions between buffers and streams throughout the lifecycle of a request. What handlers return is ready to be sent in the response.

As part of this change, we removed reliance on image-size-stream to calculate the natural image dimensions, as well as the old method for obtaining EXIF data. We now use sharp for all of this, which should represent a performance improvement.

To do before merging:

• Ensure exif data is being added to the response
• Add unit tests for exif data

Closes #390 and #393.

[coveralls]

Pull Request Test Coverage Report for Build 1221

• 108 of 121 (89.26%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.006%) to 88.101%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dadi/lib/controller/index.js	14	20	70.0%
dadi/lib/handlers/image.js	58	65	89.23%

Totals
Change from base Build 1215:	-0.006%
Covered Lines:	1640
Relevant Lines:	1797

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💛 - Coveralls