added mixed norm function
- renamed norm_mixed to mixed_norm
- use wrapper norm function instead of directly calling norm functions
- fixed column bug
- added unit tests
Merge branch 'master' of https://github.com/gbaier/cvxpy into gbaier-master

* 'master' of https://github.com/gbaier/cvxpy:
  - renamed norm_mixed to mixed_norm - use wrapper norm function instead of directly calling norm functions - fixed column bug - added unit tests
  added mixed norm function
Merge branch 'gbaier-master'

* gbaier-master:
  - renamed norm_mixed to mixed_norm - use wrapper norm function instead of directly calling norm functions - fixed column bug - added unit tests
  added mixed norm function
moved mixed norm in functions index
resolved merge conflict