validators for Unicode 

Hi, I added some validators for checking unicode input. I added:

```
IsUnicodeLetter(str string) bool  //tests if a string contains only letters, like IsAlpha but for all languages.
IsUnicodeLetterNumeric(str string) bool  //similar to IsAlphaNum but for all languages. 
IsUnicodeNumeric(str string) bool //Numbers can be 0-9 but also Fractions ¾,Roman Ⅸ and Hangzhou 〩. 
IsUnicodeDigit(str string) bool //check if the string contains only unicode radix-10 decimal digits.
```

These checks come from the go unicode package. I Added them here because they could be quite useful for validating non english input. 

The function names seem a bit long, but i can't think of anything better. Particularly they are too long for the tagMap. Suggestions?