surround string values with single quotes

Auto-completion of insert statements should generate string defaults
as '' and not "", for example:

    insert into temp_table (string_col) values ('')
instead of:

    insert into temp_table (string_col) values ("")
Merge pull request #2 from krockode/patch-1

surround string values with single quotes
register sql aliases as cell magics
Deal with ambiguous column names in select --pivot
Merge branch 'master' into reflect_keys
further work on join tab completion