[COOK-2729] - allow only http, https URI schemes

When using Windows qualified filepaths (C:/foo), the #absolute? method
for URI returns true, because "C" is the scheme.

This change checks that the URI is http or https scheme, so it can be
passed off to remote_file appropriately.
[COOK-2729] - qualify conditional w/ URI's regex

Use URI's ABS_URI regular expression to check if the URI is in fact
valid before checking if it has http or https scheme.

This catches the case of source being passed as "C:\\path\\to\file"
which may occur if someone uses the win_friendly_path in this very
module.

Or the case where someone really likes backslashes.
Merge pull request #33 from opscode-cookbooks/jtimberman/COOK-2729

[COOK-2729] - allow only http, https URI schemes