Added NetworkPresence interface, Android implementation and iOS dummy implementation
Sync takes network presence into account. Closes #157
Intent is checked against null to avoid unnecessary SunManager runs on some androids
Intent.Extras is checked against null to avoid unnecessary SunManager runs on some androids
Merge branch 'kaanjin-androidsyncimprovement' of https://github.com/toggl/mobile into kaanjin-androidsyncimprovement

Conflicts:
	Joey/Net/NetworkPresence.cs
Merge pull request #221 from toggl/kaanjin-androidsyncimprovement

Sync on Android takes connectivity into account. Closes #157