Add web proxy configuration support for linux clients network probes

Detect network availability in NetworkChecker using HTTP URL
instead of plain socket connection : this allows clients connecting
to the internet through a web proxy to configure HTTP proxy settings
for the JVM in startup shell scripts.

Adresses problem reported in OMERO user discussion forum at
http://www.openmicroscopy.org/community/viewtopic.php?f=4&t=4509
Remove logging in NetworkChecker and harmonize Insight's install doc

As per review feedback - PR 1070
Merge pull request #1070 from zeb/proxy-network-checks-develop

Add web proxy configuration support for linux clients network probes