fixed old references to images directory

all should now point to the 'assets/images/*' relative URL.

there are obviously some changes to DC content, but this issue only pertains
to fixing existing image links to match the URL pattern, and we can deal with
cutting the old content in a future issue.
closes #9
add gemfile
update readme with new dependency instructions
closes #16
reverted to using the site baseurl for assets

this does not include the custom SASS file which has a variable.
fixes #9
Update README.md

Check [this comment](https://github.com/colinsf/cft-jekyll-site/commit/08fa2812a5809bf444a01be75fec9f257a68b300#commitcomment-7651500) for more details.  I don't know how to bundle `jekyll` config values into SASS variables, so this is required for local development since we have a couple of images define in SASS.
Merge pull request #18 from colinsf/master

Fix all existing image links