This change should simplify decoder specialization by means
of parameterization in addition to subclassing.

Make it looking more uniform and easier to override if needed.

Try to reuse `SingleItemDecoder` object to leverage its caches.

The goal of this change is to make the decoder stopping on input
data starvation and resuming from where it stopped whenever the
caller decides to try again (hopefully making sure that some more
input becomes available).

This change makes it possible for the decoder to operate on streams
of data (meaning that the entire DER blob might not be immediately
available on input).

On top of that, the decoder yields partially reconstructed ASN.1
object on input starvation making it possible for the caller to
inspect what has been decoded so far and possibly consume partial
ASN.1 data.

All these new feature are natively available through
`StreamingDecoder` class. Previously published API is implemented
as a thin wrapper on top of that ensuring backward compatibility.

Also fix references to exception objects in other docstrings.

Also extend copyright to the year 2019

Debug logging refactored for more efficiency when
disabled and for more functionality when in use.

Specifically, the global LOG object can easily be used
from any function/method, not just from codec main loop
as it used to be.