specify http_proxy information as wget::fetch attribute

[rauchrob]

Currently, HTTP-Proxy information for wget::fetch can be specified by setting the top scope variables $::http_proxy and/or $::https_proxy. However, I think it is generally bad practice for a Puppet module to rely on the presence of certain global variables (if every module would do it that way, it would be a totally mess).

In my opinion, this piece of information should be specified as parameter for wget::fetch.

[carlossg]

see #61