Stringhelpers

--------------------------
Refactor of `common_subsequence()`, `longest_common_subsequence()` and `shortest_common_subsequence()`
into one function, `common_sub()`, where also `test_stringhelpers.py` is updated according to this.

* Small refactor of `reverse_order()`
* Small refactor of `odd()` and `even()`

--------------------------
New functions

* `sort()`
* `common_subsequences()`
* `longest_common_subsequence()`
* `shortest_common_subsequence()`

Small refactoring of `odd()` and `even()`.

Adds `test_stringhelpers.py`

----------------------------
Small improvements of the `reverse_order()` and `inverse()` functions.

--------------------------
Improves the `upcase_first_letter()` function.

* Any non-alphanumeric chars and spaces will now not be treated as the first letter,
which stopped the search after the first letter that should be capitalized.

--------------------------
- Fixes small typos.

--------------------------
- First release.