Decompose Attributes Into Separate Traits & Add Missing Attributes

* Several HTML5 attribute names were missing, they've been added to Attrs.
* The attributes were grouped into separate traits which can be handy for scalatags users
* Version set to 0.5.0-SNAPSHOT so as not to interfere with released versions
* ScalaVersion set to 2.11.4 to ensure modern scala compiler and compatibility with other projects
* Fixed a few documentation warnings in Core
* Added AttrPair to the Aliases in Bundle because it is a useful abstraction for scalatags clients
* Refactors trait Attrs[B,O,F] into 8 other traits that group the attributes as per HTML spec: Input, Clipboard, Media, Miscellaneous, Keyboard, Mouse, Window and Form