Asterisk 1.6 added

The ability to build versions of Asterisk 1.6 into trunk was added. Support is very preliminary, but appears to work without major issues. AstLinux will continue to produce images with the 1.4.x series until the next long-term stable release is available. We may produce images with both Asterisk 1.6.x and 1.4.x, but would consider the 1.6 versions to be beta.