Removed circular dependency between table and deleted record iterator.

When records were not loaded, the 'deleted' attribute was a RecordIterator
which referenced the table. Instead, 'deleted' is now a property which
returns a new RecordIterator every time it's accessed. Thus, the table never
contains a reference to the iterator.
Added dbversion property.
Updated changelog.
Converted dbversion property to an attribute.
Forgot to add dbversions.py.
Updated readme.
Added example dbfinfo.py.
Updated readme.
Fixed a badly written test.
Merge branch 'develop' of github.com:olemb/dbfread into develop
Rewrote examples in readme.
Reorganized readme.
Fixed some spelling in readme.
Keyword arguments are now listed with default arguments. Also minor rewrites.
Improved performance of field parsing by adding a lookup table for field types.
Refactored record iteration to make it a bit simpler and slightly faster.
Rewrote read().
Added some comments.
Improved performance of _read_record().
More speed improvements.