mysql_length doesn't work for composites

[Mike Bayer]

current API:

Index('my_index', my_table.c.data, mysql_length=10)

extend it so that columns can be passed as a dict:

Index('my_index', my_table.c.a, my_table.c.b, mysql_length={"a": 10, "b": 20})

[Mike Bayer]

diff:@a341e1c:a80bb53 master

diff:@45f8ff8:3f60069 0.8

[Mike Bayer]

Removing milestone: 0.8.xx (automated comment)