avro

AVRO-2429: Ignore unknown logical types in python2. (#687)

* AVRO-2429: Ignore unknown logical types in python2.

* AVRO-2429: Clean up math and unicode.

* AVRO-2429: Fix max precision formula.

If the maximum magnitude of a two's complement number of N bytes is:

max_mag(N) = 2 ** (8 * n - 1)

and the number of decimal digits required to represent M is:

decimal_digits(M) = floor(log10(M)) + 1

Then the maximum number of decimal digits that a N byte number can contain:

max_decimal_digits(N) = decimal_digits(max_mag(N))
  = floor(log10(2 ** (8 * n - 1))) + 1
  = floor( (8*n-1) * log10(2) ) + 1

If the biggest number has X decimal digits, the N bytes can represent all
numbers with X-1 decimal digits.

max_precision(N) = floor( (8*n-1) * log10(2) )

* AVRO-2429: Add unit tests for max precision.

Fix lint.