Use Ebib databases if present

Hi Titus,

I decided to see if consulting Ebib databases from `helm-bibtex` makes much of a difference. The results are, well, mixed. On my main laptop, which is a decent enough box with a Core i5 CPU and 8MB of memory, the difference is benchmarkable but not noticeable. With my main `.bib` file (about 1100 entries), `helm-bibtex-candidates` takes about twice as long if the entries are read from disk compared to when they are taken from an Ebib database. But execution time runs in the 1.0e-5 to 1.0e-6 seconds range, so if you’re not timing it, you can’t tell the difference. On my Atom-based netbook, the difference is again a factor of two and it is noticeable if you pay attention, but not to the extent that going through Ebib feels like a real improvement. The difference is less than a second.

So although I don’t have a larger database to test with, I think it’s safe to say that few users will really notice the difference. I suspect that even with a `.bib` file of 10,000 entries you wouldn't notice anything on a modern machine.

I’m offering the code anyway, so you can check it out and decide for yourself. Personally, I’m not convinced it’s worth complicating the code for, though.

J.