Bibfiles in TEXMF Tree

Finding bib files in the TEXMF tree seems to come up frequently---e.g. #314, #60 and #258. This seemed like a simple, clean way of implementing it: just execute `kpsewhich` with a file name and let it do the search. In the interest of performance and minimising damage to already existing projects, I've set it so that `kpsewhich` only gets run if LaTeXTools cannot otherwise find a specified bib file. If `kpsewhich` cannot be found on the path, it fails silently. I've tested this successfully on both ST2 and ST3.