Tweak Home#show view

This will ultimately house the one page app. For now, just polishing off
a few loose ends from yesterday.
Build out landing page html

Was largely a placeholder prior to this. Brought it more in line with
Evernote's landing page.
Expand html for sign in and sign up pages

Prior versions were mostly placeholders.
Initialize tag model and migrate database
Add db presence constraint on tag fields

Rolled back the db and then remigrated as I had forgotten this
constraint on prior commit.
Validate tag model and add basic associations
Generate skeleton for tagging model

This will be a join table between tags and notes.
Migrate db for tagging model
Associate models through the tagging join table
Create tags route and generate controller skeleton
Define all tags controller actions
Build basic tag views
Add list of tags to Note#show
Setup routes and controller skeleton for taggings
Allow notes to be tagged and untagged

This required a change to the routes. Taggings are now nested under
notes.
Prevent users from editing others' tags & taggings

Also DRYed out the before filter that's used to do this.
Order notebooks, tags, and notes

Notebooks and tags are ordered alphabetically. Notes are ordered by last
update.