Added the ability to specify a default bibtex bibliography

Added a new variable to specify a default bibliography in

`LaTeXTools Preferences.sublime-settings`

The variable is called `default_bibliography` and defaults to `false` (i.e. no default bibliography specified). If a `default_bibliography` is specified, it takes as input the full path to a .bib file the user wants to use by default.

The default will be used by `latex_cite_completions.py` when (1) no `.bib` file is referenced by a `\bibliography` command, or (2) Any `.bib` file fails to open (e.g. if you have `\bibliography{refs}` in your `.tex` file, but `refs.bib` does not exist in the same directory as your `.tex` file). This was added to both the completions and the command.

I apologize for what may be sloppy python. I'm very much a newbie.