SQL performance improved. Closes #403

The benefit with this commit is really interesting: from 22 queries it has been reduce to only 2, making it really fast.
Merge pull request #405 from PyBossa/issue-403-profile-slow

SQL performance improved. Closes #403
Show rank of the user in his profile page. Closes #404
Merge pull request #406 from PyBossa/issue-404-rank-position

Show rank of the user in his profile page. Closes #404
Show the Joined time in user Settings Profile
Update some screenshots in the docs.
Community Leaderboard.

This commit adds a leaderboard that can be accessed from the user profile, or directly from the Community page. It shows the best 20 users, and if the user is logged in, his current rank.
SQL query performance improvements.

This commit improves the SQL queries run within SQLAlchemy to reduce the
overhead of computing the overall progress and last activity.
SQL improvements for front page
fix critical typo in javascript template
Don't Repeat exception handling
remove more repetition of error path
SQL Improvements for Applications view.
Clean CACHE when specific events occur (new app, delete app, etc.)
Admin apps, now uses cached_apps.
Fix pagination for featured apps.
Add proper app short name to the load progress function. Closes #409
Merge pull request #407 from mk270/fix-js-typo

fix critical typo in javascript template
Merge pull request #408 from mk270/refactor-task-import

Refactor task import
factor out tree of if-statements