ENH: clean up imports and unused code

[kaczmarj]

this pull request

• removes unused imports
• removes *.pyc files
• adds a .gitignore file ignore common python files that should not be version controlled

i used flake8 and found some undefined variables in the code. i haven't fixed them in this pull request because i'm not sure what you want to do.

$ flake8 --select=F821 gopher/
gopher/modelzoo.py:1007:19: F821 undefined name 'PolyReLU'
gopher/modelzoo.py:1017:19: F821 undefined name 'Softplus'
gopher/robustness_test.py:111:34: F821 undefined name 'test_path'
gopher/add_motifs_GIA.py:52:10: F821 undefined name 'background_type'
gopher/occlude_motifs_GIA.py:17:47: F821 undefined name 'options'
gopher/occlude_motifs_GIA.py:18:88: F821 undefined name 'args'
gopher/occlude_motifs_GIA.py:19:90: F821 undefined name 'options'
gopher/occlude_motifs_GIA.py:20:13: F821 undefined name 'select_set'
gopher/occlude_motifs_GIA.py:21:10: F821 undefined name 'GlobalImportance'
gopher/occlude_motifs_GIA.py:29:50: F821 undefined name 'n_background'
gopher/occlude_motifs_GIA.py:31:40: F821 undefined name 'args'