Bug fixes and read from stdin

Revised from #12.

Fixes three bugs:

* `importing herd` and testing functions from within the python interactive interpreter executes code undesirably.  Fixed.
* Junk being passed into `--hostlist` not being sanitized like other methods.  Fixed.
* Blank list being passed into `--hostlist` not being detected as an error.  Fixed.

Adds a feature:

* Reading the host list from `stdin` if hosts.dat and `--hostlist` is not provided in the arguments.