Made uhd_images_downloader more robust to match Debian's uhd_firmware_installer

* Uses custom User Agent
* Compares md5sum of downloaded zip file to stored value
Merge branch 'maint'
UHD has man pages now

* Linux installations will automatically install man pages
* Moved usrp_n2xx_simple_net_burner and usrp2_card_burner into bin
Merge branch 'maint'