Fixes TextCorpus not having proper metadata info.

I've defaulted to returning the line number that a document appears on.
TextCorpora no longer raises exception on empty input file
Fix length for corpora on empty files
Remove commented out lines
Merge pull request #203 from cscorley/fix_textcorpus_metadata

Fixes TextCorpus not having proper metadata info (rebased)
PEP8 corpora changes
Add description to CHANGELOG.txt
Merge pull request #209 from cscorley/empty_corpora_inputs

Fix length calculation for corpora on empty files