Mapper: Correct Unknown Method Error Handling

Since refactoring in commit, 3c278975c59cf8dbcfdfdf1184e4cf9c89de7ce3
we have experienced some issues when using e.g. d-feet with banshee, as
described in BGO#725446.

This patch corrects a small mistake in the refactoring process, in the
creation of an org.freedesktop.DBus.Error.UnknownMethod message.
Merge pull request #38 from arfbtwn/banshee-bugfix

Mapper: Correct Unknown Method Error Handling
[build] Show an informative message when trying to run test from toplevel when disabled
[Protocol] Fix GCHandle usage for struct and disable fast path.

Mono compiler seem to have regressed(?) when it comes to taking the address of a generic struct type. This means we need to use boxed struct object to achieve the same thing with GCHandle.
[tests] Bump version number in introspection tests
[build] Bump to 0.8.1