Add an over-parametrized dummy coding scheme

[njsmith]

To provide one way for users who definitely want overparametrized dummy coding to just do

C(myfactor, DummyDammit)

or whatever.

[Request from Josef, talked over at PyCon]

[njsmith]

CC @josef-pkt

[kshedden]

We also need the opposite -- exclude all reference categories. This is needed for Cox proportional hazards models in survival analysis and some forms of multinomial regression. In those models the intercept is not identified, so we can't have 1 in the column-space of the design matrix.

[chrisgorgo]

+1 for optional overparametrized designs