Update documentation of proxy-uri property.

SoupSession resolve the default proxy when the proxy-uri
property is absent. Setting the proxy-uri will override the
default proxy.

This patch updates the doc to make that apparent.

https://bugzilla.gnome.org/show_bug.cgi?id=725126
Update required version of libsoup

In order to use the new SoupSession API we need 2.42 of libsoup.

https://bugzilla.gnome.org/show_bug.cgi?id=725126
Do not depend on libsoup-gnome-2.4

Everything in libsoup-gnome is deprecated.

https://bugzilla.gnome.org/show_bug.cgi?id=725126
Bump libsoup dependency to 2.34 and use default proxy resolver