通过代理运行lein.bat self-install

1，IE代理一定不要用脚本的方式：

2，修改lein.bat download部分powershell命令：
修改前：

powershell -Command "& {param($a,$f) $client = New-Object System.Net.WebClient; $client.Proxy.Credentials =[System.Net.CredentialCache]::DefaultNetworkCredentials; $client.DownloadFile($a, $f)}

修改后：

powershell -Command "& {param($a,$f) $client = New-Object System.Net.WebClient;$client.Proxy=[System.Net.WebRequest]::GetSystemWebProxy();$client.Proxy.Credentials =[System.Net.CredentialCache]::DefaultNetworkCredentials; $client.DownloadFile($a, $f)}