Kotlin port

[elect86]

Hi,

yesterday I (finished to) port exp4j to kotlin

it features a couple of things like:

• automatic build inside Expression constructor, no more ExpressionBuilder
• setting variables and function directly by passing them in the Expression constructor
• builtin variables x, y, z and w
• functions signatures compatible with others numbers other than Double (Int and Long to mention)
• added factorial to the builtin operators
• added postfix property to the operator class
• possibility to add/remove from builtin functions
• using gradle

the only downside is that is not possible to use functions with varargs arguments (right now you can define up to 9 arguments)

I ported all the tests + some of the open issues

Right now it's shaped around Kotlin, but if you are interested, I can modify it to make easier to use it from java (ie: by switching to functional interfaces) and create a PR

Interested?