Fix Maybe::operator= template definition

The assignment operator template is now enabled only if the operand is
constructible & assignable to the target contained object, so that the
template is not misused for maybe-to-maybe assignment.
Merge pull request #81 from magicant/fix/maybe

Fix Maybe::operator= template definition
Add SignalErrorCode enum class and SignalErrorCategory class
Add UnimplementedApi test helper class
Add FakeSignalNumberSet test helper class
Add FakeSignalMaskApi test helper class
Add SignalNumberSet::clone function
Add FakeSigactionApi test helper class
Add Nop class
Rename Api.cc to ApiImpl.cc

To separate object files into Api.o and ApiImpl.o. Only the latter
depends on capi.o.
Add sigprocmaskBlock & sigprocmaskUnblock functions to Api
WIP: HandlerConfiguration