DFU Library with Secure DFU v1.5.0

New features:

• Target API 26, build tools 26.0.2
• Notification channel is required on Oreo and above (a helper method to create one is here,
• Library dependency support-v4 changed to support-core-utils

Bugs fixed:

• #72 merged
• Fixed reporting extended error (561e1f2 and cf1da0f)