Source flexibility

This is PR 2 of 3 in a series of cookbook enhancements we would like to push upstream.  Hopefully you will find the changes acceptable.  All of the changes in this set of PRs should maintain backwards compatibility.  We have additional patches we would like to contribute, but they depend on the changes made in the current set of PRs.

This PR adds a checksum attribute to verify source tarball integrity (sha256), an optional url attribute that can be set to specify an alternate download location, and an install_samples attribute that controls whether or not to install the asterisk sample config files.  Finally, it moves the build location to chef's cache directory.  This is to match the prevalent convention used by other source-building cookbooks in the community.