fixed a comment bug in JS files

Hi,

I noticed there was a bug in my change, please take look at the following notes:

The extractor was extracing the following:
```javascript
angular.module("myApp").controller("helloController", function (gettext, gettextCatalog) {
    /**
     * Ignore this comment / thank you
    */
    var myString = gettext("String with no comments");
});
```
as 
```po
#.
  * Ignore this comment / thank you
    */
msgid "String with no comments"
msgstr ""
```

**Changes made**:
- Fixed the regex to look for match only at the start of the string, 
```javascript
    // Old Regex
    comment.value.match(/\/ .*/)
    // New Regex
    comment.value.match(/^\/ .*/)
```
- Fixed the empty comments picked up by extractor(only in JS files), even though no comment was inserted.

**Before**
```po
#.
msgid "String with no comments"
msgstr ""
```
**After**
```po
msgid "String with no comments"
msgstr ""
```

- Added another sub-testcase to make sure multi-line comments do not break the extractor

Sorry about that, thanks!