Add new 'use_multi_curl" option to GCM config.
Made MultiCurl usage configurable for GCM service.
Added instructions about MultiCurl usage.
Add support of iOS aps content-available option

There is a new option is aps since iOS7.
You can check that in "Table 3-1  Keys and values of the aps dictionary" here :
https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html
Merge pull request #56 from stephanecollot/patch-1

Add support of iOS aps content-available option
Suppress stream_select errors (fixes #46)
Merge pull request #54 from richsage/multicurl-optional

Make MultiCurl usage configurable for GCM