Use submodule for the reference file

[i2000s]

Just a reminder to centralize all my reference bibtex database in one place.

To do:

1. Merge the current bibtex entries to the main database at https://github.com/i2000s/Archive. Need to unify the Download link tag in the bibtemplate files in _layout (maybe with download?).
2. Set a submodule structure for this repo to use i2000s/Archive. Use a different branche for this repo.
3. Add git submodule update command to the Travis-CI code for automatic submodule updates.

The similar approach will need to be applied to other repos once this repo gets proven working fine.