Test bog-prepare-bib-file in temporary directory
Rewrite some functions to avoid find-file-noselect

Several functions relied on find-file-noselect. I hadn't realized that
it's a bad idea to use this non-interactively [1].

[1] http://emacs.stackexchange.com/questions/145/what-are-some-built-in-packages-with-nice-source-code/151#151
Rework citekey collection

- Split into two functions instead of having no-sort argument.
- Use push instead of add-to-list.
bog-citekeys-in-file: Replace add-to-list with push
bog-citekey-from-property: Cosmetic changes
Replace some cond instances with case
Remove some useless let bindings
Use org-before-first-heading-p