Use screencap bin

Use Host GPU must not be checked or the screenshot will be grey.
Fix acceptAlert on iOS 7
Merge pull request #1270 from bootstraponline/screencap_bin

Use screencap bin
Merge pull request #1273 from bootstraponline/ios7_accept_alert

Fix acceptAlert on iOS 7
Fix spacing
Merge pull request #1274 from appium/ios7_accept_alert

Fix spacing
iOs -> iOS
Remove checkMissingParams when defaults are set
Merge pull request #1276 from bootstraponline/master

Remove checkMissingParams when defaults are set