Support codings with "implicit intercept"

[vnavkal]

For some use cases, it would be nice to have patsy code a design matrix as if it were going to include an intercept (so e.g. it would exclude some categorical levels), but then not include the intercept. See #80 for details.

This pull request adds an implicit_intercept parameter to some of the public functions (dmatrix, dmatrices, incr_dbuilder, incr_dbuilders, and design_matrix_builders). When set to True, it codes the matrix as if an intercept were included but doesn't actually include the intercept.