AU: Common code to parse simple key/value store files

We use a few files, like /etc/lsb-release, that have a simple
key/value store format: each line is a key/value pair and the
key/value are separated by '='. This change make some common code to
parse (and assemble) these files, which is used in one place now but
will be used in another place soon, too.

BUG=None
TEST=attached unittests

Review URL: http://codereview.chromium.org/2105016
diff --git a/SConstruct b/SConstruct
index 1cf837a..53004ab 100644
--- a/SConstruct
+++ b/SConstruct
@@ -213,6 +213,7 @@
                    omaha_response_handler_action.cc
                    postinstall_runner_action.cc
                    set_bootable_flag_action.cc
+                   simple_key_value_store.cc
                    split_file_writer.cc
                    subprocess.cc
                    tarjan.cc
@@ -245,6 +246,7 @@
                             omaha_response_handler_action_unittest.cc
                             postinstall_runner_action_unittest.cc
                             set_bootable_flag_action_unittest.cc
+                            simple_key_value_store_unittest.cc
                             split_file_writer_unittest.cc
                             subprocess_unittest.cc
                             tarjan_unittest.cc