Changed tokenizer to accept digits that start with the deciaml separator

Hello,

I use exp4j for very simple but sometimes long equations where the values can be expressed as .002 or .1, etc. Currently the Tokenizer doesn't recognise these as numbers and throws an UnparsableExpressionException exception.

This push request adds this ability and tests cases for it, I have also added a test suite to test all test cases in one go.