don't install toplevel benchmarks package

[bnavigator]

python setup.py install and pip install control install the benchmarks module into the toplevel sitelib. This is bad practice, because it could conflict with other packages doing the same.

[coveralls]

Coverage remained the same at 89.221% when pulling 4a8cd58 on bnavigator:setup-py-package into d25ef12 on python-control:master.

[murrayrm]

Good catch.