Adding to StringUtils truncate method and test cases

Adding the following methods from `StringUtils`:
* `public static String truncate(final String str, final int maxWidth)`
* `public static String truncate(final String str, int offset, final int maxWidth)`

For example 
```
StringUtils.truncate("abcdefg", 4) = "abcd"
StringUtils.truncate("abcdefg", 6) = "abcdef"
StringUtils.truncate("abcdefg", 7) = "abcdefg"
StringUtils.truncate("abcdefg", 8) = "abcdefg"

StringUtils.truncate("abcdefghijklmno", -1, 10) = "abcdefghij"
StringUtils.truncate("abcdefghijklmno", 0, 10) = "abcdefghij"
StringUtils.truncate("abcdefghijklmno", Integer.MIN_VALUE, 10) = "abcdefghij"
StringUtils.truncate("abcdefghijklmno", Integer.MIN_VALUE, Integer.MAX_VALUE) = "abcdefghijklmno"
StringUtils.truncate("abcdefghijklmno", 0, Integer.MAX_VALUE) = "abcdefghijklmno"
StringUtils.truncate("abcdefghijklmno", 1, 10) = "bcdefghijk"
StringUtils.truncate("abcdefghijklmno", 2, 10) = "cdefghijkl"
StringUtils.truncate("abcdefghijklmno", 3, 10) = "defghijklm"
StringUtils.truncate("abcdefghijklmno", 4, 10) = "efghijklmn"
StringUtils.truncate("abcdefghijklmno", 5, 10) = "fghijklmno"
StringUtils.truncate("abcdefghijklmno", 5, 5) = "fghij"
StringUtils.truncate("abcdefghijklmno", 5, 3) = "fgh"
StringUtils.truncate("abcdefghijklmno", 10, 3) = "klm"
StringUtils.truncate("abcdefghijklmno", 10, Integer.MAX_VALUE) = "klmno"
StringUtils.truncate("abcdefghijklmno", 13, 1) = "n"
StringUtils.truncate("abcdefghijklmno", 13, Integer.MAX_VALUE) = "no"
StringUtils.truncate("abcdefghijklmno", 14, 1) = "o"
StringUtils.truncate("abcdefghijklmno", 14, Integer.MAX_VALUE) = "o"
```

Jira ticket https://issues.apache.org/jira/browse/LANG-1013
Should return StringUtils.EMPTY