add travis ci build

This pr add [travis ci](https://travis-ci.org) build configuration ( #306 )

build is for every branch/commit/pull request and for personal forks

build is failing  [![Build Status](https://travis-ci.org/enricosada/FSharp.Data.png)](https://travis-ci.org/enricosada/FSharp.Data)  because of unit tests ( see [log](https://travis-ci.org/enricosada/FSharp.Data/builds/16347834#L874) )

I tested 'build.cmd All' on windows, still failing but with less test errors.

i modified the fake script to fix compilation on mac osx ( and travis ), 
FAKE is now stable version ( not prerelease )

TODO you should do before merge:
* You should login on https://travis-ci.org/ and enable service hook for fsharp/FSharp.Data . 
After that there will be a new build every commit (see [travis help](http://about.travis-ci.org/docs/user/getting-started/#Step-one%3A-Sign-in) ) on the [fsharp/FSharp.Data](http://github.com/fsharp/FSharp.Data) repository
* on REAME.md there is the badge, should be changed to fsharp/FSharp.Data.