Seperate BibDatabase object from parser.
Add convenience methods load, loads and dump at module level.
parser's `parse()' methods now return 'BibDatabase' object.
docs and tests
Made writer into a class with a couple of attributes for configuring output.
Writer entry sorting.
Support for @string blocks and @preamble blocks (both parser and writer)
Doc updates.
Remove commented-out string replacement code
Merge pull request #33 from faph/object_vs_parser

Seperate parser from generated object