[SEARCH] Added a Cucumber scenario for searching in gem authors

    Given we now have a more complex search available
    When we search in the `author` field
    We should get some relevant results

* Added a "Searching in authors" scenario
* Added a step for creating more complex Rubygem records into the `gem_steps.rb` definition file

Use this command to run the scenario:

    $ bundle exec cucumber --name "Searching in authors" features/search_advanced.feature
[SEARCH] Refactored the search steps to a higher-level nested step "I search for ..."

Instead of repeating the low-level steps:

    When I go to the homepage
    And I fill in "query" with "<query>"
    And I press "Search"

over and over in our scenarios, we will abstract these steps to a single step:

    When I search for "<query>"

The obvious benefit is less code duplication and more readable steps.
[SEARCH] Added "search tips" sliding panel at the search results page

* Added a second form with `query` input, to duplicate the query for easier correction/change
  at the results page

* Added a HTML partial with concrete, interactive examples of queries possible with Lucene,
  hidden by default

* Added a link and JavaScript code to toggle the sliding panel with search examples

* Added CSS styling for the new elements, added a "help.png" icon from the FamFamFam suite
[SEARCH] Prevent indexing errors on Rubygem records without a version
[SEARCH] Added starting of "elasticsearch" in the Travis CI configuration
[SEARCH] Added information about installing Elasticsearch into "Contribution Guidelines"
[SEARCH] Handle search engine being not available in user-friendly way
[SEARCH] Changed, that errors when indexing to Elasticsearch are rescued

Previously, when an error occurred while saving the model into the Elasticsearch index,
the whole operation failed and an Exception has been raised.

This patch adds a `rescue` clause which logs the exception and swallows it.