An 1-bit increment of one should indicate missing compressed data

Fix a bug where an 1-bit increment of one is treated as normal
increment.

According to WMO spec 94.1.6
   The convention for representing missing data for compressed data
   within the binary Data section shall be to set the corresponding
   increments to fields of all ones.

There it should be processed as missing data.

Resolves: #33

version bump for new tables

Update tables v38, v39. Bump version

Also update talbespreparer to accomodate latest WMO tables format
changes

Bump version to 0.2.19

Support adding a preamble in the beginning of encoding output

Also bump version to v0.2.18

Bump version

Fix bug for defining and using new refvals.

Fix Python 3 test

Add Travis CI. Fix tests for Python 3.5

Some more documentation.