* Added placeholder to the base Cell class.
* Removed the \u0000 char at the beginning of the TextCell
placeholder that was there for a CodeMirror bug workaround.
* Other refactoring of Cell related Notebook methods.
* Cell insertion code completely rewritten. We now have two methods
rather than 9: insert_cell_above, insert_cell_below.
* Renaming methods to be more consistent.
* Creating new methods to provide a uniform API.
* Minor bug fixes.
* Merge/split works for all cell types.
* Notebook.select won't select an already selected cell.
* Bugs in markdown cell editing fixed.
* border-box-sizing used for Markdown cells to get correct css boxes.
* Shading/border added to Markdown cell editor.