Fixes error in NativeMethods SendMessage extern declaration

The SendMessage method causes a "no implementation" error without an
extra DllImport tag:

Additional information: Could not load type 'BotSuite.NativeMethods'
from assembly 'BotSuite, Version=1.0.0.2, Culture=neutral,
PublicKeyToken=null' because the method 'SendMessage' has no
implementation (no RVA).

Also, updated project file to reflect the removal of Settings.cs