digits shouldn't be shorter than 2 characters long

[stephenwade]

Also, prevents an infinite loop when trying to encode with a base 1 BaseConverter.

Example code that infinitely loops:

from baseconv import BaseConverter
base1 = BaseConverter('0')
base1.encode(5)

[coveralls]

Coverage decreased (-1.5%) to 98.529% when pulling 6e927f0 on stephenwade:patch-1 into 1c4fdf1 on semente:master.

[coveralls]

Coverage remained the same at 100.0% when pulling c8b7782 on stephenwade:patch-1 into 1c4fdf1 on semente:master.

[semente]

Thanks @stephenwade, it looks great!