Support "drop tables if exists"

[sqs]

Right now as best I can tell it's not possible to drop all tables registered to a DBMap if they exist.

There are 2 possible solutions:

1. Exporting the tables []*TableMap field on DbMap.
2. Adding a DropTablesIfExists method to DbMap.

Solution 1 is more flexible but solution 2 is more in keeping with the current scheme of adding a bunch of helper methods on DbMap. @jmoiron, do you have a preference or any other ideas?

---

Here is a patch for solution 2 that we use at @sourcegraph.

commit d1ea3bbd873a92d1df083f97f2e129d681f1c0a4
Author: Quinn Slack <qslack@qslack.com>
Date:   Tue Apr 22 02:27:43 2014 -0700

    DropTablesIfExists

diff --git a/dbmap.go b/dbmap.go
index 9580a10..17506ec 100644
--- a/dbmap.go
+++ b/dbmap.go
@@ -263,6 +263,20 @@ func (m *DbMap) DropTables() error {
        return err
 }

+// DropTablesIfExists iterates through TableMaps registered to this DbMap and
+// executes "drop table if exists" statements against the database for each.
+func (m *DbMap) DropTablesIfExists() error {
+       var err error
+       for i := range m.tables {
+               table := m.tables[i]
+               _, e := m.Exec(fmt.Sprintf("drop table if exists %s;", m.Dialect.QuoteField(table.TableName)))
+               if e != nil {
+                       err = e
+               }
+       }
+       return err
+}
+
 // Insert runs a SQL INSERT statement for each element in list.  List
 // items must be pointers, because any interface whose TableMap has an
 // auto-increment PK will have its insert Id bound to the PK struct field,