Add max length constraint method to StringUtil

In the *Safety time* project we need to ensure that the name of the *safetee* is no longer than a specified value. As we need to apply this constrain everywhere in the whole project I thought that it might be a good candidate for including it in the StringUtils module in the NoTils project, instead of creating our own static helper method.

- Add a new function that crops a string if its length is more than a maximum value and appends "…" at its end.
- Tests implemented

As an example:
string: thisismylongstring
maximumlenght: 5
returns this…

string: thisismylongstring
maximumlenght: 100
returns thisismylongstring