Added Category navigation

Added ability to filter by category and updated Pagination to also take
into consideration the category.
Added Category Side Menu

Created a menu containing the list of categories and added it to the
main _Layout page.
Added menu highlighting

Added highlighting to the menu so users will know what category they are
viewing.
Category Specific Page Count

When a user selects a product, the page count will be based on the
specific category.
Merge pull request #2 from Displace/Navigation

Navigation