schema_salad 4.5.20191017101802

Fix doc generation refactor t -> entry #282 by @tetron
Refactor SourceLine#makeLead #283 by @tom-tan
root the exception hierarchy in SchemaSaladException #280 by @mr-c with help from @manabuishii
base_url parameter to urljoin shouldn't be optional. #279 by @tetron
tests: do not lower case the path #277 by @mr-c
omit metaschema.py from the coverage check #275 by @mr-c