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
test/unit: fix all pep8 errors
test/unit: fix all pyflakes errors
tox.ini: run pep8/pyflakes on `test` package also

This is done so that we also ensure good style in our test code.
test/unit: better multiline string style