- added class CryptaConstant extending CryptaLeaf
- test long multiplication
- update all classes so that integers are treated as constants and in evaluation case are not added in the map of symbols
- we can do long multiplications, there is a test for that taken from issue #22
```
  SEE *
   SO =
-----
 EMOO +
MESS  =
------
MIMEO
```
- test on long division
- other thest have been taken from http://cryptarithms.awardspace.us/puzzles.html

TODO : test for BigNum and add more tests for basic cases expecially negative tests

- Added method in CryptaConstant to convert decimal integer to integer in a given base in LittleEndian
- BigNumModeler corrected method to add right integer to model in the wanted base (from config.arithBase)
- Added tests on bignum with integers
- Added tests with base change (both for bignum and normal solver)