MarcExtractor obj is now generic, not created per-record.

Macros still need to be updated to use new MarcExtractor, and their tests still failing
fix macros for new MarcExtractor pattern -- some still are doing things inefficiently although they all work
make extract_marc macro use new more efficient pattern of MarcExtractor
eliminate MarcExtractor.extract_by_spec...

convert Marc21 macros to use new more efficient pattern of re-using MarcExtractor's

This does require ugly class-instance-variable code, sorry
update README for new MarcExtractor api
fix marc_series_facet, with tests
fix demo_config.rb for new MarcExtractor pattern
Make generic line writer; use it for JSON and a debug output
Put blank lines between records in debug_writer
Add documenting comments to top of debug_writer
Added yaml_writer, too, while I'm at it
Looking at new syntax
Add tests and more useful error reporting
Move error-checking code into methods and improve error messages so you can find what the hell went wrong
isolate better errors messages
Fix string/symbol mixup in last_named_step
DRY up the argument error checking and use custom error types
Merge pull request #15 from jrochkind/debugwriter

Add a debug_writer, line-oriented output
line_writer related inline rdocs
Merge pull request #16 from jrochkind/better_parse_errors

Traject::Indexer, better error messages