New API to disable retry for an OkHttpClient.

Most applications won't want to disable retry globally. Instead, use
clone() to get an OkHttpClient for a specific, non-idempotent request,
then configure that client with the setting.

Closes https://github.com/square/okhttp/issues/1043
Merge pull request #1259 from square/jwilson_1230_fail

New API to disable retry for an OkHttpClient.