Ignore MaxLength validation when maxlength is -1

[steveschmitt]

In EF, HasMaxLength(-1) is used to indicate that no max length is defined for the field. This might be used on an nvarchar(max) field, for example.

Either

1. The "maxlength" property should be removed from the Breeze metadata during metadata generation (preferred), OR
2. The "maxlength" property should be ignored by breeze-client if it is less than zero.

Note that #1 is the preferred solution but it is a server-side change, since that is where metadata is generated.