Update default fixture to give better direction

closes #1561
- altered perspective of intro paragraph to reading from the blog
instead of from the content page
- added copy directing users to sign in to the admin area at /ghost/ and
edit the post before reading the 'Getting Started' section
- this should give the Markdown lessons better context and avoid users
getting confused as to what 'the left hand panel of Ghost' is
Remove fixed scheme from gravatar url

no issue
- removed scheme from gravatar url

Reason:
Gravatar supports ssl and the fixed scheme will cause ‚insecure
content‘ warnings.
Merge pull request #1695 from sebgie/gravatar-ssl

Remove fixed scheme from gravatar url
Merge pull request #1677 from nickpfisterer/default-fixture-update

Update default fixture to give clearer direction
Update Ghost logo image fixture
Merge pull request #1713 from JohnONolan/ghost-png