Deprecate Report#events, and implement new callbacks

[hidakatsuya]

Until 0.7:

report = Thinreports::Report.new layout: 'foo.tlf'
report.on :page_create do |e|
  e.page   #=> Object of creating page
  e.report #=> Report object
end

report.on :generate do |e|
  e.pages  #=> [page, page, ...]
end

Since 0.8:

report = Thinreports::Report.new layout: 'foo.tlf'
report.on_page_create do |page|
  page #=> Object of creating page
end

report.on_generate do |pages|
  pages #=> [page, page, ...]
end