Group inputs by destination

If a common 'dest' is defined for multiple nggettext_compile.all.files elements, the destination file will be overridden.  In the example below, the json compiled for 'second/path' will override the json compiled for 'first/path'.  This patch will group the inputs by destination before calling Compiler.convertPo().

nggettext_compile : {
  all : {
    files : [
      {
        cwd : 'first/path',
        src : '*.po',
        dest : 'json/output',
        ext : '.json'
      },
      {
        cwd : 'second/path',
        src : '*.po',
        dest : 'json/output',
        ext : '.json'
      }
    ]
  }
}