Store image meta data in arrays and adds unit tests

[achingbrain]

This pull request does a number of things (sorry):

1. Lets you store upload meta data in array properties instead of just fields (attaching plugin to an Array #5)
2. Adds a suite of unit tests to prevent future b0rkage
3. Defaults to using the system temp directory for ImageMagick temporary files (Converted files polluting defaults directory #38)
4. Adds the field name to the saved file (Overriding files when using more than one attachment field (r 0.0.4) #23)

To run the unit tests, do:

npm test

Then open coverage/lcov-report/index.html to see code coverage.

There was an existing test that looks at the output of ImageMagick to verify whether support for certain formats was enabled. I'd argue that this test should be removed as it's not testing mongoose-attachments code, rather an external dependency so can break due to circumstances out of our control, but I've converted it into a mocha unit test anyway pending further discussion.

[zeroth]

is this patch added to Master ?

[achingbrain]

No, this pull request has not been merged.

If you like, check out mongoose-crate - it works along similar lines to mongoose-attachments and supports arrays of images among other things.