Prevent dups on manual retrieve during start cache

Fixes Issue #16

If a user fired off a retrieve from the command palette during Sublime
startup caching (for a doc type that was being cached), you would wind
up with duplicates.

To fix this, the cache now implements MutableSet as well as
MutableSequence, and existing methods check for existence of duplicates.
The whole structure could use some optimisation, but will get us over
the line for now

salesforce_reference/cache.py

@@ -1,7 +1,7 @@
 import collections
 from functools import total_ordering
 
-class SalesforceReferenceCache(collections.MutableSequence):
+class SalesforceReferenceCache(collections.MutableSequence,collections.MutableSet):
     """
     A cache of SalesforceReferenceEntry objects, sorted by Title. This order
     will be maintained throughout append operations
@@ -32,17 +32,21 @@ def entries_by_doc_type(self):
 
     def __getitem__(self, key):
         return self.__entries[key]
-    def __setitem__(self, key, value):
-        self.__entries[key] = value
-        self.__maintain_cache()
+    def __setitem__(self, key, item):
+        # Enforce set behaviour
+        if item not in self.__entries:
+            self.__entries[key] = item
+            self.__maintain_cache()
     def __delitem__(self, key):
         del self.__entries[key]
         self.__maintain_cache()
     def __len__(self):
         return len(self.__entries)
-    def insert(self, key,val):
-        self.__entries.insert(key,val)
-        self.__maintain_cache()
+    def insert(self, key, item):
+        # Enforce set behaviour
+        if item not in self.__entries:
+            self.__entries.insert(key,item)
+            self.__maintain_cache()
     def __maintain_cache(self):
         self.__entries.sort()
         self.__index_entries_by_doc_type()
@@ -66,6 +70,18 @@ def __groupby(self, the_list, key=lambda x: x):
             d[key(item)].append(item)
         return d.items()
 
+    """MutableSet methods"""
+    def add(self,item):
+        if item not in self.__entries:
+            self.__entries.append(item)
+            self.__maintain_cache()
+    def discard(self,item):
+        try:
+            del self.__entries[self.__entries.index(item)]
+            self.__maintain_cache()
+        except ValueError:
+            pass
+
     """str and repr implemented for debugging"""
     def __str__(self):
         return str(self.__entries)