Implemented Minify task

Task implements minification of CSS and JS assets:
 - accepts asset file path or string source
 - tries to intelligently guess asset type
 - tries to intelligently set default destination

Code examples:
```php
$this->taskMinify( 'web/assets/theme.css' )
       ->run(); // 'web/assets/theme.min.css' default
```

```php
$this->taskMinify( ".something \n { color : red; }\n" )
       ->to( 'css/style.min.css' ) // hints css type
       ->run();
```
Output example:

```
[Robo\Task\ExecTask] running lessc css/laps.less css/laps.css
[Robo\Task\MinifyTask] Wrote css/laps.min.css (reduced by 14.8%)
```

Public methods:
 - `to( $dst )` explicit destination path
 - `type( $type )` explicit asset type
 - `__toString()` casts to minified string, without writing file


Fixes #67