indexes not working: ValueError: too many values to unpack (expected 2)
``` python
    class Meta:
        indexes = (
            (('vorname', 'nachname', 'gesamtbrutto'), True),  # Add trailing comma.
        )
```

I'm writing a small SQLite-based application with peewee 2.4.5 for which I need to import data from another souce that, sadly, doesn't supply unique IDs. Therefore I want to have my own, internal primary keys and a constraint that makes sure I don't import the same row multiple times. I'm trying to use an unique index to do so:

``` python
#!/bin/env python3
# -*- coding: utf-8 -*-
import peewee as pw

DATABASE = pw.SqliteDatabase('testcase.sqlite3')

class BaseModel(pw.Model):
    class Meta:
        database = DATABASE


class Employee(BaseModel):
    eid = pw.PrimaryKeyField()
    gesamtbrutto = pw.IntegerField(null=True)
    vorname = pw.TextField(null=True)
    nachname = pw.TextField(null=True)

    class Meta:
        indexes = ((('vorname', 'nachname', 'gesamtbrutto'), True))

if __name__ == "__main__":
    DATABASE.connect()
    DATABASE.create_tables([Employee], safe=True)
```

Sadly this doesn't work:

```
$ rm testcase.sqlite3
$ ./testcase.py 
Traceback (most recent call last):
  File "./testcase.py", line 23, in <module>
    DATABASE.create_tables([Employee], safe=True)
  File "/usr/lib64/python3.3/site-packages/peewee.py", line 2915, in create_tables
    create_model_tables(models, fail_silently=safe)
  File "/usr/lib64/python3.3/site-packages/peewee.py", line 3983, in create_model_tables
    m.create_table(**create_table_kwargs)
  File "/usr/lib64/python3.3/site-packages/peewee.py", line 3740, in create_table
    cls._create_indexes()
  File "/usr/lib64/python3.3/site-packages/peewee.py", line 3763, in _create_indexes
    for fields, unique in cls._meta.indexes:
ValueError: too many values to unpack (expected 2)
$ sqlite3 testcase.sqlite3
SQLite version 3.8.7.4 2014-12-09 01:34:36
Enter ".help" for usage hints.
sqlite> .dump employee
PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
CREATE TABLE "employee" ("eid" INTEGER NOT NULL PRIMARY KEY, "gesamtbrutto" INTEGER, "vorname" TEXT, "nachname" TEXT);
COMMIT;
```

Am I missing something here? Is this feature not available in SQLite?
