Added custom macros to Str Class

Added the ability to use custom macros for the Str Class

for example... can be used to add possessive form conversion to a string

````php
Str::macro('possessive', function($value)
{
    // append an apostrophe to indicate possession
    $value .= '\'';

    // append an s for all strings that do not end with the letter s
    if($value[Str::length($value)-2] != 's')
        $value .= 's';

    return $value;
});
````