Don't rely on the repository being initialized for sharing.

The share menu item is accessible even while the repository is still
loading; so when accessing it before loading completed there will be a
NPE.
Hardcoding the path isn't a big problem anyway as we
- do this in a lot of places anyway (mainly in the activities where the
  actual objects are loaded in the fragments) and
- we need to rely on the exact path conventions for the browse filter
  anyway
Fix downloads download.

- Fix download URL to point to the actual file and not the JSON data
- Fix download media type - zipball downloads don't work with the
  application/octet-stream type
Remove useless check.

We can't return null from getFragment() anyway, as that will lead to a
NPE in FragmentStatePagerAdapter.
Minor format fix.
Move readme HTML parsing to a background thread.

Some readmes can be very large and HTML parsing can take a lot of time
for those, therefore move as much parsing as possible into the
background. TextView.setText() will still take quite some time, but we
can't avoid that.
Fix repository search behaviour.

- Only allow user repo search if 'all' is selected as we can't search
  through a user's watched or starred repos using the search API
- Suppress error returned by Github when attempting to search through a
  user's repos while the user doesn't actually have any repos.
Resubmit search when changing search type.
Merge pull request #156 from maniac103/for-upstream

More fixes