Add kernel method

It would be nice to add kernel DMD as an option, for instance to standard DMD or streaming (or potentially TLS).  One possibility would be something like:

``` python
sdmd = StreamingDMD(kernel=10)
```

for a kernel (1 + x'y)^{10}, or

``` python
sdmd = StreamingDMD(kernel=f)
```

for a user-defined kernel f(x,y).  Any thoughts?
