Updated branding to diaspora* (text and logo)

Plus minor corrections and fixes.

@DeadSuperHero to check these before merging.

Changes:
**logo.png** 
- updated to diaspora* in Roboto bold. NB: to help smooth the edges, this file is twice the dimensions it should appear on the page. See notes for application.html.erb for this.

**application.html.erb** 
- added height and width parameters to logo link. I don't know the syntax for these in erb files, so please check and correct if necessary
- added links to tutorials (commented out until I have finished them)
- corrected link to text in buttons so this can be translated.
- changed button classes so 'Host a pod' is blue and 'Join us!' is green (this makes the 'Host a pod' button match others on the page, and distinguishes the Sign up button.

**main.css.scss**
- removed redundant uppercase
- set text in header bar to lowercase, as this looks better. NB: this currently affects also the buttons in the header bar. I don't mind this, but if you think this is awful, comment this line out for the time being until we can fix it.

**index.html.erb**
- added button under 'Choosing a pod' with link to wiki page
- changed Sign up button class to green, to distinguish it, and removed extraneous JS stuff from the old site.
- added button under 'Join the conversation' with link to getting started tutorial (commented out until tutorials finished)

**get_involved.html.erb)
- fixed problem in which line breaks in code after links left a space between text and closing full stop.

**devise.en.yml**
- improved English

**en.yml**
- changed Diaspora to diaspora* throughout.

Hope this is all OK and I've done it correctly. Please let me know if I need to make any other changes.