Build system CDN support

For each folder it is possible to define multiple hosts (to circumvent the browser limit on the number of simultaneous requests for the same host). 
For each URL we select a pseudo-random host. Each URL use always the same host.

**configuration example**

```
options: {
    buildDir: buildDir,
    srcDir: 'src/',
    assetsHome: 'assets/',
    checkDeps: true,
    depsDir: 'vendors/',
    cdn: {
        hosts: {
            'css': ['https://css1.counterwallet.dev', 'https://css2.counterwallet.dev'],
            'js': ['https://js1.counterwallet.dev', 'https://js2.counterwallet.dev'],
            'assets': ['https://assets1.counterwallet.dev', 'https://assets2.counterwallet.dev']
        }
    }
}

```