Handle encoders with a state machine

Differences between the old and new encoder methods:

Old code used string compares, New code uses an arrary lookup

Old code:

1. Tracked each change in the encoder signals
   as a movement in a direction
2. Accepted two movements in the same direction as
   movement in that direction.
3. Divided the number of changes by four to get a position

4. Old encoder made changes according to this table, which
   treats "no change" conditions 00 and 11 as movement (*).
    //directionEncoders +=_8segmentEncoder;
    // head direction
    // old   new   result old new
    //  00    00     +1    0   0 (*)
    //  00    10     +1    0   2
    //  10    11     +1    2   3
    //  11    01     +1    3   1
    //  01    00     +1    1   0
    //
    //  01    11     -1    1   3
    //  11    11     -1    3   3 (*)
    //  11    10     -1    3   2
    //  10    00     -1    2   0
    //  00    01     -1    0   1

New Code:

1. Counts each cycle through all 4 encoder values as a
   movement in a direction
2. Is able to declare a direction after one cycle
3. Does not divide by four to get a position
4. No movement is counted if input signals do not change

General Changes:

Used unsigned char where possible to save memory

Signed-off-by: Steve Conklin <steve@conklinhouse.com>
Merge pull request #22 from sconklin/for-upstream

Handle encoders with a state machine