Load plugins from /usr/lib/-directory (e.g. /usr/lib/sensact-usb.so). So remember to install plugins under /usr/lib ...

Update and clean Makefile
-so it's more like it was before merging ble_for_TI_sensortag-branch
-remove ble_sensortag-logic for now. It's now for testing purpose, but could be added later again if needed
(e.g. when linking ble-module directly in libsensact)

Update sensact.c: usb-backend registered now via plugin-manager. BLE-backend not register yet, because it's running as an
standalone process.