Rename lora mac files/classes to lorawan mac

[peggyopal]

This pull request works towards adding multihop functionality (issue #39)

Proposed Changes

• Rename the following classes:
  • EndDeviceLoraMac to EndDeviceLorawanMac
  • GatewayLoraMac to GatewayLorawanMac
  • LoraMacHelper to LorawanMacHelper
  • LoraMacHeader to LorawanMacHeader
  • LoraMac to LorawanMac
• Rename respective .cc|.h file names
• Update doc/lorawan.rst with the new class name
• Remove a duplicate include statement in end-device-status.h

[DvdMgr]

Squashed, rebased, and pushed to the develop branch.

Thank you so much for this!