spicemanager: always use isStarted() when checking if started.

There is a rare case where isStarted() == false but runner != null.

When this happens, even if a user of the library checks with
isStarted() before calling start(), he will not avoid the
IllegalStateException "Already started.".

So, always check if SpiceManager has started with isStarted().
Merge pull request #214 from gkylafas/spicemanager-always-use-isStarted

spicemanager: always use isStarted() when checking if started.