Fix 'error: a class-key must be used when declaring a friend' compile error.
Fix testcase compile failure under gcc 4.6:

google/protobuf/map_test.cc:88:16: error: expected ‘;’ at end of member declaration
google/protobuf/map_test.cc:88:18: error: ‘override’ does not name a type
.
Merge pull request #88 from tseaver/fix-class_key_must_be_declared_w_friend

Fix compiler error: "a class-key must be used when declaring a friend"
Ignore files generated from new 'map*.protobuf' sources.
Merge pull request #89 from tseaver/ignore-map_generated_files

Ignore files generated from new 'map*.protobuf' sources.