Allow custom sequence name
Cool, at first glance it looks good.  We'll need to ensure that if a pk_sequence is specified that the column is created with that sequence as well.  Lastly, do you think this is something that should be inheritable from model to model?  i.e., the database Meta attribute is inheritable, but the db_table is not.
 I've added the "read" part of this feature in 22639933f3079e6f01be173e5637672ce14f35cf but have not come up with the correct approach for "write", i.e. table creation.  Will be thinking about this, though for now should wokr.
 It should be inheritable because it has two goals.
- To let the developer set his own sequence name for each table
- To allow one sequence for all table or a group of table, like table inheritance in postgresql

I think we shouldn't allow a pk_sequence attribute in children Meta.
To lock this behavior maybe it could be interesting to raise an exception. But it could also be possible to avoid the check since the parent is found.
 Yeah, I should've updated my notes.  I've added logic to create sequences automatically when running `Model.create_table()` and to share them across models using inheritance.  It's in commit cc5d86f15dee40d45a0eacc95d346175c40eb99e

You can run the test suite using postgres:

```
createdb peewee_test
export PEEWEE_TEST_BACKEND=postgresql
python setup.py test
```

- Use Meta class to set it like
  
  ``` python
  class Meta:
      pk_sequence = 'sequence_name'
  
  ```
  
  Helpful when using a global sequence.
