Improve performances

The aim of this pull request is to reduce the CPU/memory consumption of
JSZip. The main idea is : don't transform the data if it's not
necessary.

Lazy decompress
---------------
The main new feature is the lazy decompression. If the user loads a zip
file but only read a single entry, we don't need to INFLATE the other
entries. Moreover, if the user ```generate()``` this JSZip object, we
won't DEFLATE every files : we will reuse the compressed data and only
recompress the read entry. This is the goal of JSZip.CompressedObject.
This unfortunately means that we won't be backward compatible : the data
attribute may not be calculated yet.

Don't transform the data until necessary
----------------------------------------
An other change is the type of ZipObject.data (renamed _data for the
reason above). Instead of transforming it into a string when
loading/adding an ArrayBuffer, we let it be. We will transform it on
demand (getters, generate), not before. The central part of this change
is the JSZip.utils.transformTo function. This adds a big matrix
(```transform```) to transform any supported type into any other. I
think this is worth the trouble : the other parts of JSZip just need to
know the destination type. This also means that nodejs support comes
nearly from free : the transformTo method will take care of a lot of
things.

Update the INFLATE/DEFLATE implementation
-----------------------------------------
The current implementation (from Masanao Izumo in 1999) is known to have
bugs (issues #22, #26, #29, #43, #52, #53). The new implementation is
from https://github.com/imaya/zlib.js and don't have these bugs.
This implementation is slower than the current one, but this one works.

Other possible improvements
---------------------------
The ArrayBuffer -> unicode string transformation is not efficient. On
Firefox, the TextDecoder API solves this but this is not (yet) available
on the other browsers / nodejs. A solution is to use the BlobReader
API but that means transforming all our API into an asynchronous one.