Fixed issue with UTF-8 2-byte characters

Characters were being split up and causing invalid sequences when using
`substr()`. Went ahead and updated to use `mb_strcut()` and forcing the
character encoding to UTF-8. I think the plan down the road will be to set the
internal encoding to UTF-8 but I am not currently sure how that could effect
the rest of the system (perhaps it won't). Closes #39

tests/classes/StringTest.php

@@ -86,7 +86,7 @@ class StringTest extends PHPUnit_Framework_TestCase
 	{
 		return [
 			['foo bar', 3, true,  '<span title="foo bar">foo&hellip;</span>'],
-			['foo bar', 3, false, 'foo...'],
+			['foo bar', 3, false, 'foo&hellip;'],
 			['foo bar', 7, true,  'foo bar'],
 			['foo bar', 8, true,  'foo bar'],
 		];