Treat the first Sentences of Headers as the first sentence of paragraphs

Currently I assign the Sentence.isStartParagraph value of first sentences of
section headers to true.  But we should have a independent flag such as isHeader.
Reneme Sentence.isStartParagraph to Sentence.isFirstSentence
Merge pull request #108 from takahi-i/fix-begin-space-validator-bug

Fix begin space validator bug #100