Fixed JoinSqlBuilder to include table name

The JoinSqlBuilder.ToSql() wasn't prepending field names with the
corresponding table name in the field list.
When using joins this is a must to avoid field name conflicts between
tables.
Fixed FirebirdSqlExpressionVisitor Trim support

Firebird doesn't support LTrim or RTrim.

for LTrim we need to use Trim(leading from str).
for RTrim we need to use Trim(trailing from str).

This avoids errors when executing code like this:

.Where<Northwind.Common.DataModel.Territory>(x =>
x.TerritoryDescription.Trim() == "Westboro");