add proper error state to encoder object parsing

fixes #36
Add sloppy_existing_atom as a configuration option for labels
Merge pull request #37 from macintux/friendlier-existing-atom

Add sloppy_existing_atom as a configuration option for labels
rename 'sloppy_existing_atom' to 'attempt_atom'
Merge branch 'develop' into twopointoh

Conflicts:
	src/jsx_encoder.erl
	src/jsx_to_term.erl