Fix backslashes in URLs on Windows

[dwickern]

Convert paths to use to forward slashes

Fixes errors like this:

$ goxc bintray
relative path \.goxc-temp\control.tar.gz, full path Z:\golang\bin\packer-post-processor-virtualbox-to-hyperv-xc\0.1.0\.goxc-temp\control.tar.gz
[goxc:bintray] 2016/03/24 12:04:44 Adding Header - Content-Length: 254
[goxc:bintray] 2016/03/24 12:04:44 Adding Header - Content-Type: application/x-gzip
[goxc:bintray] 2016/03/24 12:04:44 PUT to https://api.bintray.com//content/dwickern/packer-plugins/packer-post-processor-virtualbox-to-hyperv/0.1.0/\.goxc-temp\control.tar.gz
[goxc:bintray] 2016/03/24 12:04:44 Http response received
[goxc:bintray] 2016/03/24 12:04:44 Error code: 400 Bad Request
[goxc:bintray] 2016/03/24 12:04:44 Error body:
[goxc:bintray] 2016/03/24 12:04:44 Stopping after 'bintray' failed with error 'Error code: 400, message: 400 Bad Request'
[goxc] 2016/03/24 12:04:44 RunTasks error: Error code: 400, message: 400 Bad Request

[laher]

this is awesome, thankyou.

I'll add you and then you can merge it yourself.

[dwickern]

🎉