Added the option 'allow_scripts' which can be set to True for safely importing untrusted BST files.
Added a couple of safety measures into the BST file parser to prevent some egregious safety holes. If a user attempts to use the 'os' or 'sys' or 'codecs' modules, to import any module, or to attempt to open a file, then Bibulous will abort user script evaluation.
Updated documentation.
Modified the write_bblfile() method in bibulous.py to use a try-except block when writing the BBL file. This means that we should be able to write a properly formatted BBL file even when uncaught exceptions occur.