Dual-license under Apache 2 and MS-PL, with Apache as default

Future contributions must be submitted under the Apache 2 terms (which permit relicensing as MS-PL, although MS-PL does not permit relicensing as Apache 2).

Contributors who released their code exclusively under the MS-PL will need to sign off on this change.
Merge pull request #598 from imazen/apache-license

Dual-license under Apache 2 and MS-PL, with Apache as default