Clean up all temp files created during processing and storage

[achempion]

stavro/arc#285

from @dmarkow

Fixes #256

Arc creates temp files in the directory that System.tmp_dir returns, but never cleans up after itself. Running unchecked on a production server could result in running out of disk space. These temp files are created in three scenarios:

1. The input is binary data
2. The input is a remote URL
3. During transformations for each version

Scenarios 1 and 2 seem very similar (converting something that isn't a local file path into a local file path), however scenario 1 was happening once per transformation (where 10 versions/transformations meant the binary data would be written to disk 10 separate times), whereas scenario 2 was happening once right at the beginning (the URL is retrieved and saved to disk immediately during Arc.File.new/1).

Scenario 3 creates a temp file for each transformation.

Changes:

• Write binary input to a tempfile one time during Arc.File.new/1 rather than once per transformation (the tradeoff is that even with no transformations, it'll still write once, but this matches how remote URLs are handled)
• Got rid of Arc.File.ensure_path as it was only there to convert binary data to a path which now happens automatically
• Add a tempfile? attribute to %Arc.File{} that can be used later for cleanup (I preferred this over just testing the path to see if it's in the temp dir, otherwise there's a chance of deleting the input file itself if it was also in the temp dir)
• After storing each version, the version's temp file is deleted (scenario 3)
• After storing all of the versions, the input temp file is deleted if it exists (scenarios 1 and 2)
• Added tests for checking temp files (had to change some of the setup/teardown logic in the tests, but they all pass including the S3 tests)

[achempion]

@dmarkow @kolorahl what do you think?