add get_security_file() function to utils.path...
add get_security_file() function to utils.path
This makes it easier for third party tools to retrieve security files
(e.g. json connection files) by name and [optional] profile name alone, without
knowledge of IPython's directory structure.
For example, whenever ipkernel outputs:
` --existing kernel-12345.json --profile foo`
that file can be found with:
get_security_file('kernel-12345.json', profile='foo')
