Removed vim swap file.

* Sneaky little guy has been sitting there for a month without me
  noticing.
Tiny refactor in signatures.
Added external atomic file implemenation.

* Pulled from repo at https://github.com/sashka/atomicfile.
* I have contacted the author and he says he will license his code
  tomorrow. If it ends up that we can't use this we'll want to drop
  this commit.
Fixed error handling code in filetransfer.

* We would try to delete a file we have open already.