- Add some big-integer algorithms.

- Add some big-integer algorithms.
- Improve names of BigIntegerUtils conversion functions.
- Make stringToBigInteger recognize a leading + sign.
- Remove false claim that BigIntegerUtils provides istream >> operator.
- Improve BigUnsigned shift operators: now just one version that handles a
  positive or negative int shift distance.
- Throw exceptions on /, % by zero (just not divideWithRemainder).
  - Apply previously forgotten cleanups to BigInteger /=, %=.
- Start a testsuite.  There's only one test.
- Clean up Makefile a bit.
- Improve comments in sample.cc.