Add search to find and view public tags and gists across all profiles

Fixes #7

 * Generate SearchController with index action
 * Refactor search route
 * Update bootstrap-sass gem
 * Add 'Search' to main navigation
 * Refactor MyGists::Search to receive either a Profile instance or profile username String
 * In the views, be more semantic by using a definition list for listing gists
 * Add Profile.usernames to get all usernames for search typeahead
 * Add ActsAsTaggableOn::Tag.public_names to get all public tag names for search typeahead
 * Define protected SearchController#search to search public gists when search params exist
 * Add Github.profile_page to return the link to a specific profile on GitHub
 * Define various SearchHelper methods for checking existence of, formatting and getting search terms
 * To simplify search index view, create search form and header partials
 * In search index view, if a search exists, show the search terms and the results
 * Style search section
 * Override Bootstrap typeahead active selection to have white text instead of black text
 * Move gists and header styles into layout SASS file since the selectors are used across sections