Add docs for manual sequence counter management
Remove automagic pk-based sequence setup

Related to issues #78, #92, #103, #111, #153, #170

The default value of all sequences is now 0; the automagic
``_setup_next_sequence`` behavior of Django/SQLAlchemy has been removed.

This feature's only goal was to allow the following scenario:

1. Run a Python script that uses MyFactory.create() a couple of times
   (with a unique field based on the sequence counter)
2. Run the same Python script a second time

Without the magical ``_setup_next_sequence``, the Sequence counter would be set
to 0 at the beginning of each script run, so both runs would generate objects
with the same values for the unique field ; thus conflicting and crashing.

The above behavior having only a very limited use and bringing various
issues (hitting the database on ``build()``, problems with non-integer
or composite primary key columns, ...), it has been removed.

It could still be emulated through custom ``_setup_next_sequence``
methods, or by calling ``MyFactory.reset_sequence()``.