Hardware bringup updates

Updates for bringing up Piksi hardware:
* Add function in AcqResult class to return the mean of the best acq SNR for all acquired satellites.
* Add option in Flash.erase_sector arguments to not check for restricted sectors.
* Refactor code that downloads index and firmware files from download.swiftnav.com.
It doesn't make sense to tell the user to check that they're running the latest firmware.  That is what they're trying to update, and it wouldn't cause a download to fail.