Added .travis.yml file.

I added a travis.yml file for Travis CI.
In case you don't know travis it is PURE AWESOME.
On every code check in (and even pull request) it'll run the unit tests
and will show you if the tests still fail.
The test are just run on one platform (ubuntu linux x64) but on multiple
perls (5.8 .. 5.18). The results are published in near-real-time and you
can even let travis notify you via IRC, mail, or whatever you want.

Travis is integrated with Github, so in github you can see for pull
requests if tests would still pass etc.

See for results of my test repo:
https://travis-ci.org/mbeijen/DBD-mysql-1

Also, DBI and DBD::ODBC already use Travis.

How to activate Travis for DBD::MySQL:

 - go to https://travis-ci.org/
 - sign in using Github account
 - Click on 'accounts', switch on integration for DBD-mysql repo
 - Accept this pull request

Done! Results will come in. See them in real-time in the dash, if
you want.