Bitarray

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* add `decodetree` object, for speeding up consecutive calls
to `.decode()` and `.iterdecode()`, in particular when dealing
with large prefix codes, see 103
* add optional parameter to `.tolist()` which changes the items in the
returned list to integers (0 or 1), as opposed to Booleans
* remove deprecated `bitdiff()`, which has been deprecated since version
1.2.0, use `bitarray.util.count_xor()` instead
* drop Python 2.6 support
* update license file, 104

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* add optional index parameter to `.index()` to invert single bit
* fix `sys.getsizeof(bitarray)` by adding `.__sizeof__()`, see issue 100

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* add PyType_Ready usage, issue 66
* speedup search() for bitarrays with length 1 in sparse bitarrays,
see issue 67
* add tests

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* support signed integers in `util.ba2int()` and `util.int2ba()`,
see issue 85
* deprecate `.length()` in favor of `len()`

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* Use `Py_ssize_t` for bitarray index. This means that on 32bit
systems, the maximum number of elements in a bitarray is 2 GBits.
We used to have a special 64bit index type for all architectures, but
this prevented us from using Python's sequence, mapping and number
methods, and made those method lookups slow.
* speedup slice operations when step size = 1 (if alignment allows
copying whole bytes)
* Require equal endianness for operations: `&`, `|`, `^`, `&=`, `|=`, `^=`.
This should have always been the case but was overlooked in the past.
* raise TypeError when trying to create bitarray from boolean
* This will be last release to still support Python 2.6 (which was retired
in 2013). We do NOT plan to stop support for Python 2.7 anytime soon.

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* add more tests
* C-level:
- simplify pack/unpack code
- fix memory leak in `~` operation (bitarray_cpinvert)