Create a function taking a positive integer as its parameter and returning a string containing the Roman Numeral representation of that integer.

Modern Roman numerals are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero. In Roman numerals 1990 is rendered: 1000=M, 900=CM, 90=XC; resulting in MCMXC. 2008 is written as 2000=MM, 8=VIII; or MMVIII. 1666 uses each Roman symbol in descending order: MDCLXVI.

**Example:**
```rust
solution(1000); // should return 'M'
```
**Help:**

```
Symbol    Value
I          1
V          5
X          10
L          50
C          100
D          500
M          1,000
```
Remember that there can't be more than 3 identical symbols in a row.

More about roman numerals - http://en.wikipedia.org/wiki/Roman_numerals