Use nullptr instead of nsnull

nsnull has been removed from libxulrunner. Since we compile in C++11 mode
anyway, we can use nullptr instead.
Make compatible with xulrunner 18
Drop workaround for missing hashbytes function

Use correct archives and linker flags to include the HashBytes function built
into XULRunner.

Kudos to megaloni.

Fixes #12.
xpcom_abi:get return values with NS_ERROR_GET_CODE

Since Gecko 19, the type "nsresult" is not an integer, but an enum class.
This causes compiler errors when returning the nsresult from a function.

By wrapping nsresult in the NS_ERROR_GET_CODE() macro, we can retrieve the numerical value of the error code.
Since Debian testing still has 10.0 (without mozglue) we need to conditionally compile for it.
implement readonly attribute boolean isLoggedIn
Have GetIsLoggedIn always return TRUE as per https://bugzilla.mozilla.org/show_bug.cgi?id=839961#c40
conditional compile only if GetIsLoggedIn is defined
move all this conditional-compilation crap to another file
in git-describe, only match main tags, not side tags like debian/* which does its own thing
add some more conditional compilation stuff to make things work for firefox 27+