Add cell array of strings utilities.

Two new utilities are provided:

`cellStrCat` - Creates a new cell array of strings from a list of cell arrays
of strings by concatenating each cell of input i+1 with each cell of input i.

```
>> cellStrCat({'a','b'},{'1','2'},'_',{'x','y'})

ans =

    'a1_x'   'a1_y'   'a2_x'   'a2_y'   'b1_x'   'b1_y'   'b2_x'   'b2_y'
```

`num2cellStr` - Converts a numerical array (of any dimensions) into a cell
array of strings of the same dimension.

```
>> num2cellStr(1:4)

ans =

    '1'    '2'    '3'    '4'
```
Add help for `cellStrCat` and `num2cellStr`
Merge pull request #50 from avalenzu/cell-str-utils

Cell array of string utilities