Added a Travis-CI configuration file.

Initial stab at a first Travis config file.

Travis is used for Continuous Integration testing.

Here is an example of the current result of the builds:
https://travis-ci.org/JoakimSoderberg/Libevent

Each build is done using both gcc and clang.

Currently CMake is building with the following:
 - default settings
- OpenSSL turned off.
- Threads turned off

Autotools uses only the default settings. 

As you can see from the results, the CLang build with OpenSSL disabled currently fails:
https://travis-ci.org/JoakimSoderberg/Libevent/jobs/17558347

Due to this test:

```
The following tests FAILED:
	 41 - test-ratelim__group_lim (Failed)
```

An extremely useful tool!

You should probably setup your own Travis account for the official repository.

Travis will also work with github hooks, so you will get build status on all pull requests and such (as you see below, there's a red cross on this one because of the above mentioned problem.

It is also possible to add a small image to the README.md file to get a quick status on the project github page: ![Build status](https://travis-ci.org/JoakimSoderberg/Libevent.png)