Report accurate tested-doc count

[chalin]

• Fixes htmltest inaccurately reports doc count instead of tested-doc count when docs are ignored #227
• Fixes Tests don't take ignored-docs into consideration, and have misleading comments #228
• Document store: adds DocumentCount() and IgnoredDocCount() functions
• Reports the number of tested docs "out of" the full doc count when some documents are ignored
• Adjusts TestDocumentStoreIgnorePatterns test to check tested-doc count vs just the doc count, and adds a clarifying comment that IgnorePatterns doesn't affect stored document count

[chalin]

/cc @markmandel - FYI, since this is (in a sense) a followup to #181 -- and thanks for that fix Mark!