Case insensitive uniqueness

```
from mongoengine import *

connect('dbname')

class A(Document):
    b = IUniqueStringField()

c = A()
c.b = 'whatever'
c.save()

d = A()
d.b = 'WhatEVER'
d.save()
```

results in:

```
(env)[mongoengine] python test.py                                                                                                                             19:00:59 git:(master*)
Traceback (most recent call last):
  File "test.py", line 11, in <module>
    c.save()
  File "/Users/wojcikstefan/Repos/mongoengine/mongoengine/document.py", line 266, in save
    raise NotUniqueError(message % unicode(err))
mongoengine.queryset.NotUniqueError: Tried to save duplicate unique keys (E11000 duplicate key error index: tumblelog.a.$_b_lower_1  dup key: { : "whatever" })
```