FIX: add nullable tag for sessionConfiguration

Change-Id: Id4e8bf24f6207be5c5f3831d380c2e3736d3405a

MOD: bump version to 3.0.5

Change-Id: If264f170372928cf0ca0d884fec419977859d471

ADD: event accessory and convenience method for starting request with…

… willStart willStop and didStop event

Change-Id: Ia1bac70bde5e3f761d4c62c6002209d51e7c59c9

MOD: update YTKNetwork to 3.0.3

Change-Id: Id0d064807ab7bbe9ca572150f99f4b2fb84ff5d4

FIX: crash when download path is nil and not init cache folder

Change-Id: I36968d0f307049b51642c53f0cfbccdafa53b1ab

MOD: update source to gerrit.zhenguanyu.com

Change-Id: I8656bcb7189b155c8d718c28bf5406f37f1a1583

MOD: update readme and fix typo

Change-Id: I9d10ad8f2f9b5876311868dd1bc3ff92aa7db829

MOD: use <> for import headers of other module

Change-Id: I8020263ceeb563978569ee9f16ac115e32576d19

MOD: update version to 2.1.3

Change-Id: I6d3abd96184baa2e3dff47247979bb3d8f262489