Toolbar and Selection Fix

- Toolbar is created once.
- Bind `mousedown` to elements to detected where the selection started.
- Fix checkSelection and prevent toolbar to open in a invalid editor.
- Display block on toolbar when activate() is called

disableToolbar and data-disable-toolbar behavior:
- If `disableToolbar` is true on options, all elements in that instance of mediumEditor will have toolbar disabled. (no toolbar will be created)
- if `disableToolbar` is false on options:
 - if all elements have `data-disable-toolbar` toolbar will be disabled in each one (no toolbar will be created)
 - if any element dont have `data-disable-toolbar`, a toolbar will be created and will work only on that element.
- Fix check if selection element is valid

+ Plus fix bindPaste's paragraph creation.
Merge pull request #86 from pedronasser/fix-check-selection-toolbar-disable

Fixes selection and disable toolbar behaviors
Add Multi-editor Demo
Enable Multi Editor in same or many instances.

- Improve getSelectionElement() to work in any cases and fixed the bug on multi element selection.
- Fix Issue https://github.com/daviferreira/medium-editor/issues/88 & https://github.com/daviferreira/medium-editor/issues/89
- Refactor the fix for https://github.com/daviferreira/medium-editor/pull/86 (sorry for that but I don’t understand why to bind mousedown is required. This execute checkSelection() twice. So I tried to merge both code for a better and clean code. Hope it’s ok)
- Improved binding for multiple editor instance. checkSelection() is called only one time per instance.
- Improved allowMultiParagraphSelection filter by removing empty tags elements before counting and consider header tags has a paragraph too (same as medium).
Merge pull request #90 from devalnor/feature/multieditor

Feature/multieditor: Enable Multi Editor in the same (or many) instance.
1.3.0
Merge pull request #2 from daviferreira/master

1.3.0