Made the default parser language configurable

**Feature:** Configurable default parser language
  As a feature writer
  I want to set a default feature file language
  So that I do not have to write # language: xx in every file

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I have some troubles running the unit-test suite, even on a clean checkout of `develop`. See the specific error below.
Also I am unsure how to write an effective unit-test for this simple change. Suggestions are welcome.

This PR is related to https://github.com/Behat/Behat/pull/280

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```
1) Warning
The data provider specified for Tests\Behat\Gherkin\ParserTest::testParser is invalid.
Unable to parse in "...behat\/gherkin\/tests\/Behat\/Gherkin\/Fixtures\/etalons\/multiline_name_with_newlines.yml" at line 7 (near "  Feature description").
```

Judging by the Travis CI build, it seems like there is a compatibility problem between the Gherkin library and Symfony 2.1 and up. Symfony 2.0 seems to work just fine.