Add automated testing by Travis CI 

# Motivation

Recently, we've met some issues

  - [Issue 127: Block dependency does not work](http://code.google.com/p/lz4/issues/detail?id=127)
  - [Issue 128: fullbench.c typo runs 2 lz4hc tests as lz4](http://code.google.com/p/lz4/issues/detail?id=128)

I believe we can detect this kind of issue by automated testing.
As a first step, I'd like to introduce Travis-CI.


# How to enable Travis-CI

 - Open https://travis-ci.org/
 - Click 'Sign in with GitHub' in top right corner.
 - Open profile page : https://travis-ci.org/profile
 - Enable seesaw switch of target repository (lz4).
 - Return to home https://travis-ci.org/
 - Wait a minute. You'll see build log. ([My example](https://travis-ci.org/t-mat/lz4/builds/23125692))