Iterator: Ignore start/end if it's the empty string (or empty buffer)
clean up LD_RETURN_CALLBACK_OR_ERROR calls
Merge remote-tracking branch 'origin/start-end-as-empty-string'

Conflicts:
	test/iterator-test.js
moar tests for empty iterator start/end
compactify LD_STRING_OR_BUFFER_LENGTH
macro aesthetics (80-char \'s)
replace some macros with static inline functions
version bump for @0.2.1, update changelog