post_to_liquid_raw plugin: update to Jekyll 1.2.1

Convertible#to_liquid now requires optional arg `attrs`.
Add static_file_modified plugin
Add convertible_lf_fix plugin
tags plugin: URI.escape fix

Delete quick fix for mojombo/jekyll#1211 and add quick fix for
mojombo/jekyll#1568.
Gruntfile.js: add `watch` task

* Add "watch" task (alias of esteWatch)
* "default" task now builds before watching
README.markdown: update version info (Jekyll 1.0.3 -> 1.2.1)
Upgrade Jekyll to 1.2.1

* post_to_liquid_raw plugin: update to Jekyll 1.2.1
* Add static_file_modified plugin
* Add convertible_lf_fix plugin
* tags plugin: URI.escape fix
* Gruntfile.js: add `watch` task
* README.markdown: update version info (Jekyll 1.0.3 -> 1.2.1)