implement KaitaiStruct as context manager

[dgladkov]

Followup to #6. KaitaiStruct now implements context manager interface, so you can use with keyword like with any other resource. Examples of context manager usage with automatically closed resource:

>>> with KaitaiStruct.from_file(filename) as ks:
...     print ks._io._io.closed
... 
False
>>> ks._io._io.closed
True

>>> myio = KaitaiStream(open(filename))
>>> myio._io.closed
False
>>> with KaitaiStruct(myio) as ks:
...     print ks._io._io.closed
... 
False
>>> ks._io._io.closed
True

This is generally accepted way to deal with resources that need to be closed in Python. However, you can create instance of KaitaiStream manually, but then you have to remember to close the resource.

>>> ks = KaitaiStruct.from_file('setup.py')
>>> ks._io._io.closed
False
>>> ks.close()
>>> ks._io._io.closed
True