Save iOS screenshots to /tmp/appium-ios-screenshots/

Fix #983

Fix instruments dir
Merge pull request #997 from bootstraponline/fix_ios_screenshot

Save iOS screenshots to /tmp/appium-ios-screenshots/
Change to make Android requests more generic and allow install, uninstall and isInstalled to be called without having serial id defined.
Merge pull request #1000 from snevesbarros/abd_fix

Change to allow isInstalled, install, uninstall to be called for Android without providing serial id