Intermediate progress towards cleaning up Spans.  Span is now an immutable value.  SpanVar is immutable.  MutableSpanVar is mutable.
Progress on NerSpans from the NER DocumentAnnotators.
Not all Ner variables are labeled.  NerLabel renamed NerTag; ("Tag" will now refer to a CategoricalVar associated with a Token.  New variable LabeledConllNerTag, etc.  Correctly name NerTag from Conll2003 as Iob, not Bio.
Simplify LabeledVars by not distinguishing between "WithTarget" varieties and the others.  All LabeledVars now have target variables.
Rename PosLabel to PosTag, inkeeping with NerTag change.  Also renaming files appropriately.
Make more NER taggers add NerSpanLists attr to Document.
Merge and fix conflicts.
Merge pull request #83 from andrewmccallum/intermediate-span-changes

SpanVar, POS, NER, LabeledVar simplifications