New syntax features

[Bykiev]

This is a port from NCalc https://github.com/ncalc/ncalc/pull/38:

• Added support for positive sign with numbers
• Added support for numbers with trailing dot
• Added support for case-insensitive logical operators
• Added support for using ** as POW alternative

Closes #56

[sklose]

looks good to me, thanks for your contribution @Bykiev