Add reading_time property to posts.

The property returns a rough estimate of the reading time for the blog
post in minutes.  This code is inspired by
https://gist.github.com/zachleat/5792681.

Note: This commit does not make any changes to the templates
Update bootstrap_helper.tmpl

Changes include:
1. set CSS from BootstrapCDN to the 3.0 release (instead of RC2)
2. change // to http:// (breaks if you test it locally with
   file://)
Don't fail on missing dependencies for rest extensions (Fix #629)
Merge branch 'master' of github.com:ralsina/nikola
support parametrized and custom pygal styles (Fix #628)
Split post_pages into POSTS and PAGES (Issue #601)
add support for rel=self links in generic_rss function (not used yet)
Use rel=self link for main RSS feed
Fix #264
Fix #647
we don't support python 2.5 anymore
Compile less files directly to the output dir, instead of theme dir.
Merge pull request #651 from punchagan/fix-less-dst

Compile less files directly to the output dir, instead of theme dir.
Merge pull request #646 from ralsina/rel-self-rss

Improve RSS feed interoperability
Fix bundles when using less files.

Change the file_deps to the output folder, since #651 changed the
build_less to write files directly to output directory, and copy
assets should've already run, when we are building bundles.
Add explicit task dependency on copy_assets for build_bundles
Merge pull request #652 from punchagan/fix-less-dst

Fix bundles when using less files.
Merge branch 'master' of github.com:ralsina/nikola into kill-post_pages
fix tests
backport test changes from another branch