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.
This file is still catching URLErrors.  Without the import these will just become NameErrors.
It's probably better to import URLError from update_downloader now since that's where it's raised.