top_n, floor_date, custom sql joins, and full method spec

Fixes

• filter now preserves column order, rather than moving grouping columns to left (#205)
• symbolic representations now correctly align on keywords (#222)

Features

• sql supports custom join conditions via sql_on (#202)
• siuba.series.spec now includes all Series methods, even unsupported ones (#209)
• the spec also now is derived from the file siuba/series/spec.yml (#211)
• siu Symbolic is no longer falsey (#210)
• added new verb top_n (#222)
• added vector functions ceil_date and floor_date to siuba.experimental.datetime (#222)

QA

• re-enabled testing of example jupyter notebooks (#206)