Tests for FastMoney overflows

Tests for various edge cases in FastMoney:

 * `FastMoney#multiply(Number)` looks broken in
   #cc64343967f8aa38002513f1e99f6ff905660f72
 * `FastMoney#divide(Number)` throws a `ArithmeticException` even if the
    result would fit into FastMoney
 * `FastMoney#multiply(Number)` used `#doubleValue()` which lost precision
   fixed by #cc64343967f8aa38002513f1e99f6ff905660f72
 * tests for cases that would have silently overflown before
   #cc64343967f8aa38002513f1e99f6ff905660f72
 * `FastMoneymultiply(long)` did not use the exact method from `Math`
   and was silently overflowing

This commit contains two failing tests points 1 and 2 above.
Merge pull request #51 from marschall/overflow-tests

Tests for various edge cases in FastMoney:, provided by marschall.