Improve virtualenv support & egg-link resolution

- add sys_path= kwarg to Script & Evaluator constructors

- store sys_path for each evaluator instance

- replace get_sys_path with get_venv_path

- get_venv_path: use addsitedir to load .pth extension files

- get_venv_path: look for egg-link files in all directories in path

test/test_evaluate/venvs/venv34/lib/python3.4/site-packages/smth.py

@@ -0,0 +1,6 @@
+import sys
+sys.path.append('/path/from/smth.py')
+
+
+def extend_path():
+    sys.path.append('/path/from/smth.py:extend_path')