Use foobar2000's HTTP interface

Implement getUrl() in terms of SDK\http_client.h. Removing the
dependency on the WinINet API makes it possible to compile the
project with a free Express edition of Visual Studio.
Remove get_item_default_path()

The contextmenu_item_simple::get_item_default_path() method is
obsolete since v1.0. With a newer version of foobar2000, implementing
this method causes the plugin to be listed under "Legacy Commands".
Update readme.txt

Change the SDK version from 0.9 to 1.1.
Code cleanup

Fix indentation, whitespace, etc.