Split exporter in to for ipynb->ipynb
some trailing whitespace
import baseexporter in init
Exporter -> TemplateExporter / BaseExporter
fix 1 test
renamed:    exporter.py -> template_exporter.py
renamed:    baseexporter.py -> exporter.py
renamed:    template_exporter.py -> templateexporter.py
Rebase changes made by hand
Check if class is an instance of Exporter, not TemplateExporter
Fixed Travis, missing import in export.py
Relative import (suggested by @MinRK)
add Cell.is_mergeable method

and check it in the Notebook merge methods
HeadingCells are not splittable or mergeable

closes #4200
Revert b23f5b1525e76d5a
Merge pull request #4079 from minrk/status_started

trigger `Kernel.status_started` after websockets open
protect against unavailable tornado in post_processors.init
update reveal docs with recent changes
add nbconvert serve exclusions without tornado
install tornado in travis