Add coverage for unit tests

Coverage:
 * Run the unit tests with coverage and show a command line report like this:
```
Code Coverage Report 
  2014-05-07 06:16:51

 Summary: 
  Classes: 46.67% (7/15)
  Methods: 18.52% (10/54)
  Lines:   60.36% (102/169)

\NordSoftware\Paytrail\Common::DataObject
  Methods:  50.00% ( 1/ 2)   Lines: 100.00% (  1/  1)
\NordSoftware\Paytrail\Common::Object
  Methods: 100.00% ( 3/ 3)   Lines:  84.62% ( 11/ 13)
\NordSoftware\Paytrail\Http::Client
  Methods:  55.56% ( 5/ 9)   Lines:  71.74% ( 33/ 46)
\NordSoftware\Paytrail\Object::Address
  Methods:  20.00% ( 1/ 5)   Lines:  55.56% (  5/  9)
\NordSoftware\Paytrail\Object::Contact
  Methods:  12.50% ( 1/ 8)   Lines:  61.11% ( 11/ 18)
\NordSoftware\Paytrail\Object::Payment
  Methods:  16.67% ( 2/12)   Lines:  57.14% ( 28/ 49)
\NordSoftware\Paytrail\Object::Product
  Methods:  12.50% ( 1/ 8)   Lines:  53.33% (  8/ 15)
\NordSoftware\Paytrail\Object::UrlSet
  Methods:  20.00% ( 1/ 5)   Lines:  55.56% (  5/  9)
```
 * Send coverage to Coveralls. This gives you reports over time like this: https://coveralls.io/r/hugovk/paytrail-php
 * Added Coveralls coverage badge

Travis CI builds:

 * In addition to PHP 5.4, added Travis CI builds for PHP versions 5.3, 5.5. 5.6 and HHVM. 
 * PHP 5.3 and HHVM are "allowed failures": they fail but it doesn't cause the whole build to fail
 * (PHP 5.3 fails because: `facebook/webdriver v0.2 requires php >=5.4.0 -> no matching package found.`)
 * (HHVM fails because: `Xdebug is not loaded.`)

TODO:
 * Before accepting PR, go https://coveralls.io/repos/new and add the nordsoftware/paytrail-php repo.
 * After adding repo to Coveralls/accepting PR, change the two instances of `hugovk` to `nordsoftware` for the coverage badge in README.