Support for rotary encoder

[martinfelis]

Adds support for rotary encoders (potential fix for #11). Two pins can be linked by specifying the same id for the rotenc_idoption e.g.:

    [raspberry-gpio]
    enabled = true
    bcm21 = volume_down,active_low,10,rotenc_id=vol,step=1
    bcm20 = volume_up,active_low,10,rotenc_id=vol,step=1

In principle multiple rotary encoders should be supported, simply use different rotenc_ids.

In case you want to use the reverse rotation simply swap the events in the first argument to the bcm specification.

[Gadgetoid]

Should be:

from mopidy_raspberry_gpio.rotencoder import RotEncoder

Tests pass locally for me with this change.

[codecov-io]

Codecov Report

Merging #12 (289edf8) into master (b4f6137) will increase coverage by 3.67%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   91.08%   94.76%   +3.67%     
==========================================
  Files           2        4       +2     
  Lines         101      172      +71     
==========================================
+ Hits           92      163      +71     
  Misses          9        9              

Impacted Files	Coverage Δ
mopidy_raspberry_gpio/frontend.py	93.75% <100.00%> (+2.08%)	⬆️
mopidy_raspberry_gpio/rotencoder.py	100.00% <100.00%> (ø)
mopidy_raspberry_gpio/__init__.py	100.00% <0.00%> (ø)

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[martinfelis]

@Gadgetoid thanks for the hint.

[Gadgetoid]

Thanks for the awesome addition to this library!

Now to... uh... figure out how to release this sucker. It's been a while.

[martinfelis]

In case it is possible to add this somehow: the contributions of this pull request was joint work with @bombadil.