POM: use new bsd_2 license, not custom resolver

The license-maven-plugin 1.6 adds BSD-style license templates:
https://jira.codehaus.org/browse/MLICENSE-80

With this fabulous new addition, we can eliminate our custom
license resolver in favor of the stock bsd_2 license.
Update license and file headers

This was done using:
  mvn license:update-project-license license:update-file-header

The only real change is the loss of the legal disclaimer at the bottom
of the license. This disclaimer is not actually part of the BSD-2
license proper, but rather an addendum. For relevant discussion, see:
  https://jira.codehaus.org/browse/MLICENSE-80