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

jedi/evaluate/__init__.py

@@ -61,6 +61,7 @@ that are not used are just being ignored.
 """
 
 import copy
+import sys
 from itertools import chain
 
 from jedi.parser import tree
@@ -79,7 +80,7 @@ from jedi.evaluate import helpers
 
 
 class Evaluator(object):
-    def __init__(self, grammar):
+    def __init__(self, grammar, sys_path=None):
         self.grammar = grammar
         self.memoize_cache = {}  # for memoize decorators
         # To memorize modules -> equals `sys.modules`.
@@ -88,6 +89,13 @@ class Evaluator(object):
         self.recursion_detector = recursion.RecursionDetector()
         self.execution_recursion_detector = recursion.ExecutionRecursionDetector()
         self.analysis = []
+        if sys_path is None:
+            sys_path = sys.path
+        self.sys_path = copy.copy(sys_path)
+        try:
+            self.sys_path.remove('')
+        except ValueError:
+            pass
 
     def wrap(self, element):
         if isinstance(element, tree.Class):