support for scenario descriptions

Hey all,

cucumber supports a description underneath a scenario.

For example

```
Feature: This is my feature name

In order to be a good steward
As a user of the system
I want to test my scenario

    Scenario: This is my scenario name

        Here is part of my description. Normally you will
        probably just use the name, but it is valid Gherkin to
        follow the name with a multiline description

        Everything up until the first Given, When, or Then will be
        slurped up into the description of the scenario.

        Given that I have a thing
        And the thing is shiny
        Then show the thing to someone
```

`behave` version 1.2.2 does not support this though, and raises an exception when it reaches the first line of the scenario description.

For example

```
Failed to parse "/home/tarupp/features/BehaveTest.feature":
    Parser failure in state steps at line 9
```

This pull request includes enhancements that will allow `behave` to read the description (to prevent parser errors) as well as store the description in a description field.

For example

```
{'background': None,
 'description': [u'Here is part of my description. Normally you will',
                 u'probably just use the name, but it is valid Gherkin to',
                 u'follow the name with a multiline description',
                 u'Everything up until the first Given, When, or Then will be',
                 u'slurped up into the description of the scenario.'],
 'feature': <behave.model.Feature object at 0x1759a50>,
 'filename': '/home/tarupp/features/python/BehaveTest.feature',
 'keyword': u'Scenario',
 'line': 7,
 'name': u'This is my scenario name',
 'steps': [<given "that I have a thing">,
           <given "the thing is shiny">,
           <then "show the thing to someone">],
 'tags': []}
```