Fix pages on Windows

[koppor]

This fixes #1105.

On "Git Bash" on Windows, the dpl executable runs cmd.exe, which does not play well with the bash syntax. Thus, I added a check to be on Windows and then execute bash.exe.

This required

• new dependency on os to check for Windows
• reroute calls from Kernel.system to shell

I also fixed the setup phase. The git config command was called inside the source directory (which typically is not a .git directory) and not in the target directory (which is the git repository being pushed).

Tested on Windows with a GitHub repository.

Minor fix: typo

[koppor]

fa80748 also shows the change of the order. The issue is that git config --global is not used and thus the git config command was executed in the directory which should be uploaded. That directory is not necessarily a git directory: With dpl, I can upload arbitrary directories to gh-pages.