Add contentPath attribute

[alexspeller]

This adds a contentPath attribute that you can provide instead of a getCellContent method. I found that a lot of the time I just wanted a named attribute from a row rather than any formatting, and this allows that less verbosely. As it is a path, it's possible to do multilevel access e.g. contentPath: 'foo.bar.baz'.

Of course, there is still the option to provide the function to do anything more advanced e.g. formatting etc.

(I am working towards #31 but want to do a bit at a time with some minor api changes first, as it will change how I need to refactor those features)

[ppong]

hey @alexspeller I definitely agree with you on this. I had been thinking about doing something similar also in the Ember.Table.DynamicTableCell example,
https://github.com/Addepar/ember-table/blob/master/examples/table_with_dynamic_bar/table_bar.coffee

[ppong]

lgtm merged