Remove print statements from Matlab unit tests
Add methods isctime and isdtime for LTI objects

This change makes
   >>> sys.isctime()
an alternative to the existing function
   >>> isctime(sys)
Its advantage is that it avoids checking the type of `sys`, and
is a little cleaner: LTI objects now know whether they are
continuous or discrete time, and the isctime() function no longer
needs to poke around inside the LTI object to determine this.
Add method issiso for LTI objects
Implement c2d without slycot (fixes #23)

Conversion from continuous-time to discrete-time are now implemented
as methods of the TransferFunction or StateSpace classes.  The
function sample_system() is still provided, and takes either a
state-space system or transfer function as an argument.

The implementations simply call the routine `cont2discrete` from
`scipy.signal`

This new organization into methods avoids some messy type checking in
sample_system(), and also avoids some unnecessary conversions between
state-space systems and transfer functions, which fixes #23.