Raise better IOError if a connection file is not found

when starting parallel.Client

Example:

    IOError: Connection file u’~/.ipython/profile_default/security/ipcontroller-client.json' not found. Is a controller running?
set contentType='application/json'

instead of skipping straight to headers

avoids jQuery's crazy JSONP detection on `=.*??`,
which was triggered because it assumed the contentType was unspecified,
and thus should be guessed based on content.
test saving with problematic cell contents
even better IOError for no connection file
require less >= 1.7.5

fixes bug where `vbox > *` style was applied to `vbox()` classes
Merge pull request #6087 from minrk/better-error-no-connection-file

Raise better IOError if a connection file is not found
Use low overhead object heritence in Js (Object.create vs new)

the use of

    XX.prototype = new YY();

Does trigger the constructor of YY without the nead for it. `Object.create`
does go around this limitation and target browser that are relatively Old.

Cf https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create

for more info

It might help to get rid of some logic in constructors that check wether some
options are passed in, that were causing errors on noteboko load.  Typically
`if(element){ }` on Abstract  `Cell` constructor.
Merge pull request #6699 from Carreau/low-overhead

Use low overhead object heritence in Js (Object.create vs new)
Merge pull request #6697 from minrk/less-1.7.5

require less >= 1.7.5
Merge pull request #6693 from minrk/please-no-jsonp

set contentType='application/json'
move the navbar-collapse transition to xs

avoids stacked navbar on narrow (but not too narrow) windows
collapse the navbar on narrow windows

so it doesn't take up a bunch of vertical height when not in use
tweak collapsed navbar style

- notification and indicators appear above menu on small screen
- collapsed menu button floats left
move grid-float-breakpoint to 540px

collapses navbar before running over to two lines
due to help/indicator collision.