- slightly better support for bind params as column clauses, either

- slightly better support for bind params as column clauses, either
via bindparam() or via literal(), i.e. select([literal('foo')])
- removed "table" argument from column().  this does not add the column
to the table anyway so was misleading.
- Select _exportable_columns() only exports Selectable instances
- Select uses _exportable_columns() when searching for engines
instead of _raw_columns for similar reasons (non selectables have no engine)
- _BindParamClause no longer has a _make_proxy().  its not a ColumnElement.
- _Label detects underlying column element and will generate its own
column()._make_proxy() if the element is not a ColumnElement.  this
allows a Label to be declared for nearly anything and it can export
itself as a column on a containing Selectable.