Adds pysqlite package to test requirements
Add unit tests for AssociationDict
Adds support for AssociationDict type for POST

The issue is that AssociationDict type does not have an `append` method
Add test for unique key (integrity) violation
Add rollback on integrity exception

Without the rollback any further writes to the database would fail.

issue #158
Return 404 on empty search result

An empty result should return a 404 not a 400
Return a sinlge object if relationinstid is given in API.get
Modify the order of postprocessor
String to dates for relations

When updating the related nested models with the date fields, they (the date fields) don't convert to the `datetime` python objects and it raises an exception. For example, we have such models:

```
class Computer(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.Unicode, unique=True)
    purchase_time = db.Column(db.DateTime)
    notes = db.relationship('Note')


class Note(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    text = db.Column(db.Text)
    created_at = db.Column(db.DateTime, default=datetime.datetime.now)
    computer_id = db.Column(db.Integer, db.ForeignKey('computer.id'))
```

We send such PUT request to /api/computer/18:
```
{
    "id":18,
    "name":"Galina.",
    "notes": [
        {
            "text":"ew",
            "computer_id":18,
            "created_at":"2013-11-23T13:18:57"
        }
    ],
    "purchase_time":"2013-11-09T14:55:23"
}
```

The `purchase_time` field is being perfectly parsed and converted to the `datetime` python object in the `.patch()` method of view, but the `created_at` field in `notes` raises an exception:
`StatementError: SQLite DateTime type only accepts Python datetime and date objects as input. (original cause: TypeError: SQLite DateTime type only accepts Python datetime and date objects as input.) 'INSERT INTO note (text, created_at, computer_id) VALUES (?, ?, ?)' [{'text': u'test text', 'created_at': u'2013-11-23T13:18:57', 'computer_id': 18}]`
Merge pull request #265 from bildja/patch-1

String to dates for relations
Merge pull request #264 from yoyicue/master

Modify the order of postprocessor.
Merge pull request #260 from B4dM4n/get_relationinstid

Return a single object if `relationinstid` is given in `API.get()`.
Merge pull request #246 from Svenito/test-requirements

Adds pysqlite package to test requirements
Adds changes from previous commits.
Prepares for release 0.12.1.
Bump version number to 0.12.1
Set development version number to 0.12.2-dev
Mark test requirement for pysqlite in setup.py.
Removes unnecessary imports from setup.py.
Adds necessary import removed by previous commit.