Fix an iOS 7 crash that occurs when you background the application and have finish_sync_in_background enabled.

iOS 7 kills the application if you attempt to endBackgroundTask on a task that's already ended.

Also, fix a compiler ambiguity where the argument was named application and there was an instance variable with the same name.
Remove rho_sync_issyncing definition since we are calling issyncing now.
Make rho_sync_issyncing call issyncing() and also check if m_pImpl is NULL.

In RhoConnect, issyncing calls rho_sync_issyncing (a C function it seems), which works on the version of LLVM I have installed with Xcode 4.x, but the LLVM packaged with Xcode 5 tries to call rho_sync_issyncing as a class method. That class method then calls rho_sync_issyncing, which it interprets as a class method rather than the C function, so it gets into a loop where it eventually crashes.

This fix WILL NOT completely fix this bug. You will also have to modify Classes/impl/initRhoConnectClient.cpp in the RhoConnect client gem and comment out the class definition of rho_sync_issyncing (probably also need to do it in the header).
NSLog the result of rho_rcclient_issyncing to help with debugging.
Merge pull request #265 from ziggythehamster/rhodes-4-ios7-fix-crash-on-background

[iOS 7] Fix crash when using RhoConnect and finish_sync_in_background or checking if the RhoConnect client is syncing