add a post method to create a new slice

[vemonet]

It is actually really messy to create a new slice
http://www.bioontology.org/wiki/index.php/Virtual_Appliance_FAQ#How_do_I_add_or_change_slices.3F

So here is just a little post method to create it using a HTTP POST on the API

Query example:
curl -X POST -H "Content-Type: application/json" -H "Authorization: apikey token="" -d '
{
"acronym" : "my_slice",
"name" : "My Slice",
"description" : "A nice slice",
"ontologies" : ["ONT1", "ONT2", "ONT3"]
}' http://data.my-bioportal-appliance/slices

[vemonet]

Add 3 method to create, patch and delete Slices from ontologies_api.
See https://github.com/sifrproject/ontologies_api/blob/lirmm/controllers/slices_controller.rb#L34:L56 to get the synchronize slices with groups method

[alexskr]

superseded by #87