TravisCI: init SauceLabs

Here I hook TravisCI with [SauceLabs](https://saucelabs.com/), which test against nearly every browser.
You may check **tests/unit/RunSauceLabs.hx** for the list of browsers I enabled for testing. 

SauceLabs kind of encourage each project to have its own SauceLabs account.
I opened a sub-account for haxe under my own account. Let me know if you wanna change it or something.

Also keep in mind that since it is using TravisCI's [secure environment variables](http://docs.travis-ci.com/user/build-configuration/#Secure-environment-variables), which is not available for pull requests, so SauceLabs testing will not be run for pull requests. Our original testing using nodejs would still be run for such cases. And once a pull request is merged into **HaxeFoundation/haxe**, TravisCI will run the test once more which SauceLabs will be available. Well, better late than never.

Note that the unit test is currently failing on IE 6/7/8, so I didn't enable them.

IE6:
```
Test.hx:240: Generated at: 2014-01-26 20:35:07
Test.hx:242: START
TestEReg.hx:24: should be null
TestEReg.hx:32: should be null
Test.hx:224: ABORTED : [object Error] in unit.TestMisc.testJSon
Test.hx:227: STACK : undefined
Test.hx:224: ABORTED : [object Error] in unit.TestSpecification.testInt32_unit_hx
Test.hx:227: STACK : undefined
TestDCE.hx:157: unit.ThrownWithToString should have member field toString
Test.hx:187: DONE [4217 tests]
Test.hx:329: SUCCESS: false
```

IE7:
```
Test.hx:240: Generated at: 2014-01-26 20:35:07
Test.hx:242: START
TestEReg.hx:24: should be null
TestEReg.hx:32: should be null
Test.hx:224: ABORTED : [object Error] in unit.TestMisc.testJSon
Test.hx:227: STACK : undefined
Test.hx:224: ABORTED : [object Error] in unit.TestSpecification.testInt32_unit_hx
Test.hx:227: STACK : undefined
TestDCE.hx:157: unit.ThrownWithToString should have member field toString
Test.hx:187: DONE [4217 tests]
Test.hx:329: SUCCESS: false
```

IE8:
```
Test.hx:240: Generated at: 2014-01-26 20:35:07
Test.hx:242: START
TestEReg.hx:24: should be null
TestEReg.hx:32: should be null
TestDCE.hx:157: unit.ThrownWithToString should have member field toString
Test.hx:187: DONE [4258 tests]
Test.hx:329: SUCCESS: false
```