Fix a lexicon parser crash.

Fixes #64. The parser was crashing due to an empty "tr" element. The remaining
oddities in the text mirror the entry displayed on Perseus, with one
exception: the "key" value, which we use to display the entry heading, is
incorrect. We need to use the "orth" value instead. I have created issue #65
to track this bug.
Use "orth" instead of "key" for lexicon headings.

Addresses #65. This fixes the headings in the lexicon entries, but we're still
using the key values for lexicon lists. We need to either fix the original
text or modify the GRDBC to use orth values instead of key values.
Update manifest and readme for v1.2.1 release.