url_helper functions don't correctly apply $baseURL

[lonnieezell]

The path portion of $baseURL is not applied correctly base_url() and system_url() functions.

Example:
$baseURL = 'http://example.com/blog/';

helper('url');
echo base_url();

// Outputs http://example.com
// Should be http://example.com/blog

Looks like the methods should use URI::resolveRelativeURI() to join the paths. And ensure the path is set correctly by IncomingRequest.

Discovered via #148