Remove type conversions

[yin]

User can do type-conversion directly after fetching flag value. This feature added bulk of hard-to-test code, which is likely not to be reused nor maintained in future.

    String userID = arg_user.get();
    User user = UserResolver.find(userID);

Custom flag type were avoided by gflags. All parsing should be done after fetching the string value in the client code.

[yin]

Done in v0.3.0-beta1, but the alternative way of defining new flag-types must be documented before closing this out.