added optional `name` argument to `exe` property

The new job description property `exe.name` is introduced by the patch
It allows user to explicitly set a name for executable (the value of `ARGV[0]`)
The property is optional, if not supplied, the default method for setting `ARGV[0]` will be used
Currently the default is to set it to "node name"
Docs updated.
Merge pull request #89 from pkit/exe_name

added optional `name` argument to `exe` property