Bump to 0.14.0 to reflect divergence from stable
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Alternative PNG alpha-channel extraction

According to PNG spec filtering happens on a per-byte basis and uses
corresponding bytes from other pixels. This means that we don't have to actually
unfilter all the data. Instead it's possible to split pixel data into planes.

A few points to note:

* Filters are reused from original image
* Resulting opaque PNG uses the same filtered data
* Resulting mask PNG image uses the same filteres data
* Both resulting images are valid PNG images
* All the unfiltering masth is skipped which results in great boosts

Here are some benchmarks:

RBX
==========================================================

Original
--------

                   user     system      total        real
opaque:        2.724633   0.541629   3.266262 (  1.698649)
transparent: 340.380027   9.747552 350.127579 (326.535695)

Mem: 225-227 MB

Modified
--------

                   user     system      total        real
opaque:        2.675154   0.526945   3.202099 (  1.706802)
transparent:  77.626282   0.724561  78.350843 ( 72.843238)

Mem: 95-97 MB

MRI
=========================================================

Original
--------
                   user     system      total        real
opaque:        1.900000   0.190000   2.090000 (  2.206105)
transparent: 339.740000   6.330000 346.070000 (349.369761)

Mem: 110-130 -> 150-180 MB (Leak?)

Notes: Mem fluctuates widely. Median steadily rises. Mem leak?

Modified
--------
                   user     system      total        real
opaque:        1.780000   0.170000   1.950000 (  1.964678)
transparent:  50.380000   0.350000  50.730000 ( 51.129041)

Mem: 36-38 MB

JRuby
=========================================================

Original
--------
                   user     system      total        real
opaque:        5.390000   0.270000   5.660000 (  2.817000)
transparent: 564.830000   5.690000 570.520000 (474.866000)

Mem: 287.4 -> 298, 293 -> 312.2 MB (Leak?)

Modified
--------
                    user     system       total         real
opaque:         5.190000   0.210000    5.400000 (   2.497000)
transparent: 3559.290000  10.180000 3569.470000 (3614.186000)

Mem: 197.9 -> 211, 185.2 -> 187.2 MB (Leak?)

                    user     system       total         real
opaque:         5.440000   0.240000    5.680000 (   2.985000)
transparent: 3553.670000  12.020000 3565.690000 (3668.508000)

Mem: 224, 223.1 MB

Notes: Mem steadily rises, no fluctuation. Sometimes it drops.
Use the same bit depth for alpha channel as for colors
Rename the method to reflect what it actually does
Fixed Image/PNG specs
Removed unused `pixel_bitlength` method