Issue #1707: Improves stringByRemovingScriptsAndStrippingHTML performance

After a profiling session, i found out that the following method was performing extremely time-consuming operations:

**Class: NSString+Helpers**
```
 - (NSString *)stringByRemovingScriptsAndStrippingHTML;
```

The most lengthy OP is the NSRegularExpression allocation itself. Let's minimize that by using a singleton instead.

**Update:** 
This executes in a BG thread. Nevertheless, its performance can be dramatically improved.