Removed range detach calls

In line with https://github.com/timdown/rangy/commit/601b80c2bcc8f0c65bf233660f9e3bff5bbe9c18
Improve content.extractContent() method

Allow document fragments to be passed in.
Add comments for getContent() and cursor.before() and after()
Factor out getInnerHtmlOfFragment() in content.js
Return only content in cursor.before() and after()

Previously the common ancestor was included in the documentFragment
that before and after returned as well. Now it is easier to understand
what is happening.
Remove duplicate code

Editable is already made global in editable.suffix
Update Changelog
Add beforeHtml() and afterHtml() to Cursor

These are just convenience methods that return strings
instead of document fragments.
Add content.createFragmentFromString()
Add appendTo and prependTo to editable
Update default behaviour
Update Changelog and dist dist files
Merge pull request #80 from upfrontIO/expose-merge-methods

Add appendTo() and prependTo() to API