Fix some obvious bugs in Entry.to_bib
Add tests for class Entry
Move core of parse_month to algo
Some simple example scripts
Accept an iterable of files or strings to parse in succession
examples: Simplify examples using new iterable parse interface
Separate crossref resolution from database retrieval

Previously, retrieving the database checked crossrefs, but did not
resolve them, and crossrefs were later resolved one entry at a time.
This unnecessarily prevented callers that didn't care about crossref
resolution from retrieving a syntactically correct database, and also
made crossref resolution more difficult than necessary and less
flexible than possible.

This changes the API so the database can be retrieved without crossref
checks and combines crossref checking and resolution into a separate
whole-database operation.
Give example file name for complete command line
Enforce BibTeX-like ordering on crossrefs
Warn about nested crossrefs, like BibTeX does
Implement min_crossrefs, like BibTeX -min-crossrefs
examples: Use --min-crossrefs in bib2bib example
Implement a simple format-based name pretty printer
examples: Better author formatting for bibparse example
examples: Remove unused import from bib2bib
examples: Perform error recovery in bibparse