Add custom user-agent support

We default to the WebKitGTK+ user agent with Epiphany and the version
appended to it.

This can be overriden/modified in two ways:

- The gconf key /apps/epiphany/general/user_agent has the maximum
  priority, and if it's set it will be always used.

- If the gconf key is not set, vendors can still customize the UA by
  placing a branding.conf file in the share data directory. One
  example would be:

  [User Agent]
  Vendor=SUSE
  VendorSub=11.2
  VendorComment=2.29.94-1

  And the result would be:

  Mozilla/5.0 (X11; U; Linux i686; fr-fr) AppleWebKit/531.2+ (KHTML, like Gecko)
  Safari/531.2+ SUSE/11.2 (2.29.94-1) Epiphany/2.29.94

Bug #580903

Signed-off-by: Xan Lopez <xan@gnome.org>