maintain instances of exporters for use in export_by_name

avoids recreating new Exporters every time
add basic logging to NbConvert stages
don't reload template on every export

only load template if self.template doesn't exist
more default_config into Exporter._config_changed
propagate new values to singletons in export_by_name
tweak support-file output

move individual file output to debug level
add bytes-written
docstrings
warn on glob with no matches
don't allow 'template' to specify 'template_file'

must use template_file explicitly
remove wrong support file log message
add log statement to PDFPostProcessor
clear template when template_file changes
generate `export_foo` methods

and populate `exporters` dict mapping short names to classes
rollback singleton exporter instances in `export_foo`
create single exporter in NotebookApp

rather than using export_by_name
Merge pull request #3846 from minrk/nbconvert_log

Add basic logging to nbconvert