Use double instead of float to store Float fields in MySQL

The precision of MySQL float is not enough to handle all Python floats.
From http://docs.python.org/tutorial/floatingpoint.html:
"...almost all platforms map Python floats to IEEE-754 “double precision”"

trytond/backend/mysql/fields.py

@@ -55,7 +55,7 @@ class Float(fields.Float):
 
     @staticmethod
     def sql_type(field):
-        return ('float', 'float')
+        return ('double', 'double')
 
 
 class Numeric(fields.Numeric):

trytond/backend/mysql/table.py

@@ -175,7 +175,7 @@ class TableHandler(TableHandlerInterface):
                         ('varchar', 'text'),
                         ('text', 'varchar'),
                         ('date', 'timestamp'),
-                        ('bigint', 'float'),
+                        ('bigint', 'double'),
                         ('int', 'bigint'),
                         ('tinyint', 'bool'),
                         ('decimal', 'numeric'),