Update dialect to account for unsupported features

[binarymatt]

There are several items inherited from the PGDialect that this dialect needs to override. Let's record them here and then update the dialect:

• [ ] remove enum support
• [ ] remove sequence

[scribu]

I'm having trouble with an auto-incrementing column:

class Article(Base):
    article_id = sa.Column(sa.Integer, primary_key=True)

The SQL that SQLAlchemy generates looks like this:

CREATE TABLE articles (
  article_id SERIAL NOT NULL

but it should look something like this:

CREATE TABLE articles (
  article_id int IDENTITY(0, 1)

http://docs.aws.amazon.com/redshift/latest/dg/r_CREATE_TABLE_examples.html

[graingert]

@scribu please move this issue to https://github.com/sqlalchemy-redshift/sqlalchemy-redshift