string::assign appversion to alephversion and enhance prefs unit tests

Existing Prefs unit tests didn't catch an append bug in libchrome's
file_util::ReadFileToString().  Change the get/set tests to verify idempotence,
the Prefs tests now fail without a fixed libchrome.
Merge pull request #54 from vcaputo/copy_appversion_test_idempotence

string::assign appversion to alephversion and enhance prefs unit tests