Implemented generate thumbnail and burstPdf

`burstPdf`: Splits a pdf into pages
`generateThumbnail`: generates png thumbnail
Properly delete temp files in burst,thumbnail
Refactored code to cleanly delete temp files

Previously, some temp files were not deleted under certain error
conditions, or simply not deleted altogether. This would lead to
hosts running out of storage space in a high-volume prod situation.
DRY out code and give temp files meaningful names

In the event that things mess up and temp files are accumulating on one
of our servers, it will be nice to know which function is causing it.
Style fixes
Gave convert -resize a percent instead of abs size
Changed function signature to reflect buffer API
Added a size argument to generateThumbnail
Made burstPdf return in page order, with page nums

Return object format:
{
  page: page number
  file: file buffer of page
}
Updated package.json
Removed testing harness
Merge pull request #4 from graysonc/image-thumbnails

Image thumbnails and some bugfixes