source/base: Make it possible to get the underlying sd_event_source

source/base: Only disable the event if it exists

This shouldn't happen in practice but will be needed to supporting
moving.

source/base: Take an event directly instead of sdevent

Also expose a mechanism for retrieving our internal event.

event: Add trivial functions

clock: Add class for future use

event: Allow extracting the event and sdevent impl

sdref: Moves should actually invalidate the reference

This also makes it possible to use in boolean expressions to check
validity.

test/event: Fix extraneous Event creation

source/base: Add the prepare callback functionality

source/base: Expose trivial built in methods

Large refactoring

makefile: Clean up compiler flags

event: Add watchdog functions

source: Add class

Designate some headers as internal

Initial Commit