add travis build

Hi, this pr add build file for https://travis-ci.org

build every commit is:
- compile with xbuild on osx mono 3.2.5 
- run unit tests

Example log of this commit build: https://travis-ci.org/enricosada/fsharp/builds/15941628
Example badge for README: [![Build Status](https://api.travis-ci.org/enricosada/fsharp.png)](https://travis-ci.org/enricosada/fsharp)

let me know if you need more info about it