Added hex numbers support

[michalf1]

No description provided.

[VitaliyMF]

This PR cannot be accepted:

• existing tests are not passed, no tests for expressions with hex-numbers
• this code can lead to 'index-out-of-range': if (s[lexem.End] == '0' && s[lexem.End + 1] == 'x') {
• motivation to have hex-numbers in expressions is not explained. Hex constants are not typical, and when they are needed it is easier to provide custom function in the evaluation context to parse them, something like this:

varContext["ParseInt32"] = (Func<string,int>)((s) => {
  int numConst;
  if (!Int32.TryParse(lexem.GetValue(), NumberStyles.HexNumber, CultureInfo.InvariantCulture, out numConst)) {
    throw new Exception(String.Format("Invalid number: {0}", lexem.GetValue()));
  }
});
Console.WriteLine(lambdaParser.Eval("ParseInt(\"0xFF\")", varContext));

[michalf1]

Hello Vitaliy,
I don't agree that hex constants are not typical (I use them quite often). In my opinion the parser is incomplete without that functionality. Anyway I made some fixes and tests - now it's passing all tests.
Br,
Michał