Adds more flexibility to :new_page task

Page filenames/directories are lowercased and sanitized by default to
allow a greater flexibility:

- new_page["/path/to/file"] # => source/path/to/file/index.markdown
- new_page["path/to/file"] # => source/path/to/file/index.markdown
- new_page["my path/my file"] # => source/my-path/my-file/index.markdown
- new_page["lorem.ipsum.dolor"] # => source/lorem-dot-ipsum.dolor
- new_page["Questions & Answers"] # => source/questions-and-answers/index.markdown
Add custom date format to pages, add 'updated' field again

Reverts changes of c2a68cc where I accidentally removed support for 'updated' field, see comments of issue #164 for details.
Add custom date format %o for ordinal representation of the day
Date format: Remove unnecessary escapement
Fix custom colors -bg-back, closes #269
Make third-party asides default to false, closes #263
Added missing default qualifiers to some sass variables.
Provide a way to configure font family for subtitle.
Make header font size customizable.

Change $header-font-size variable to specify size.
Merge pull request #288 from jakemcc/octo-head

Make header font-size customizable.
Add missing $header-padding-top, $header-padding-bottom.
Add header font size and padding to custom layout
Remove !default from property values
Introduce distinction between preview/productive site generation

Posts which contain the YAML attribute `published: false` are usually
not generated by Jekyll. With this patch they can be previewed just
like published posts on localhost using `rake watch`or `rake preview`.

NOTICE: Before pushing to the productive environment, use `rake generate`
to update the public directory and remove posts which are flagged not to be
published.
Improved, I think, the Titlecase plugin
Set flag in preview mode and re-generate files before deployment if necessary
nested lists should not have a margin-bottom
Merge pull request #298 from nistude/master

Nested lists should not have margin-bottom
use correct .rvmrc syntax
Merge pull request #315 from vamsee/master

rvm does not recognize the current .rvmrc syntax