Improve iPhone simulator system root sdk selection. Code cleaning.

If sdk version is, for example, `7.0.3` and available system root sdk is `7.0` then `[DTiPhoneSimulatorSystemRoot rootWithSDKVersion:sdkVersion];` returns `nil`. To fix that we are now additionally looping through all sdks in `[[ISHDeviceVersions sharedInstance] allSDKs]` and comparing `shortVersionString` with sdk version.

Also removed useless imports and class definitions in `ISH*` headers and fixed import order and spaces usage in some other files.
Merge pull request #345 from facebook/nekto/fix_sdk_system_root

Improve iPhone simulator system root sdk selection. Code cleaning.