Include write isolation level in SqlStorageOptions

Hi, we had a problem with HangFire defaulting isolation level to Serialized, so I have made some changes to HangFire.SqlServer so the write job isolation level can be configured as part of SqlServerStorageOptions

original problem: http://discuss.hangfire.io/t/hangfire-sqlserver-defaults-write-isolation-level-to-serializable/224

Let me know what you think.

Thank you