Update mention of tweaks.php in README.md

I first thought my download was broken, but I believe this file is no longer in use.
Updated with mention of extras.php instead
Added strip_tags when get_the_title is used for title attribute in image.php

Aaddresses _s issue 184 - https://github.com/Automattic/_s/issues/184
Skip printing categories widget in 404 template if there's only one category

Addresses issue 183 - https://github.com/Automattic/_s/issues/183
Make sure that the theme's style enqueue handles are properly prefixed.
Update README.md with search and replace steps for handles.
Revert part of previous commit so that search and replace step 3 is parsed properly.
_s: Add a Post Format Archives heading to archive.php. Props @sixhours,
@philiparthurmoore. Fixes #95.
Merge pull request #188 from slobodan/issue-184

Added strip_tags when get_the_title is used in title attribute in image.php. Fixes #188.
_s: Use the_title_attrinute() when we don't need to specify a post.
See #188, closes #184.
Merge pull request #180 from pierrepavlov/patch-1

Update mention of tweaks.php in README.md
_s: /inc/etras.php is active by default.
Corrected if statement formatting
_s: Remove vendor prefixes on box-shadow and some legacy IE6/7
adjustments. Props @SeamlessThemes. Fixes #176.
Merge pull request #189 from slobodan/issue-183

Skip printing categories widget in 404 template if there's only one category.
_s: Minor updates to categories widget. Closes #183.
_s: Revert partial heading change.
_s: Simplify the way we do responsive images. Props @philiparthurmoore.
Fixes #120.