Kernel cache: Store if we have the source for a kernel

Then don't try to estimate kernel source size if we don't have it:
In fullKernelSize() only call add the source code if we have them,
i.e. did not drop it earlier, because doing so will crash on OSX 10.9.
This should fix issue #21
Merge pull request #24 from gicmo/osx_kern_source

Only estimate kernel source size if we actually have the kernel source