Support foreign keys

[diegonc]

This add supports for foreign key table constraints on the server side.
A new parameter was added to SQL.Server::createTable which specifies the table constraints to be used in the following format:

[ [ <constraint type> <constraint options>]
  ... ]

Currently the only constraint supported is FOREIGN KEY with the following inputs:

constraint type = '$foreign'
constraint options = { $key: [<column1>, ...],
                       $ref: { $table: <table name>,
                               $cols: [<foreign column 1>, ...] }}

But new ones can be added easily by extending the SQL.Sql::_Constraints object with new functions from options to a SQL clause.