Prepare for publishing to Maven Central

* Inherit from org.sonatype.oss:oss-parent:7
* Fixed SCM URLs. The connection URL was invalid, and the 'git' protocol
  is not suitable for the developer connection. I chose to use SSH.
* Using maven-assembly-plugin as the standard way of creating
  distros. Set up resources and removed maven-resources-plugin and
  maven-antrun-plugin configurations.
* Added buildnumber-maven-plugin and enhanced configuration of the
  maven-jar-plugin to get proper build info into the manifest. Since
  this plugin creates a timestamp as well, the other properties for
  timestamps were removed.
* Added property 'project.build.sourceEncoding' in order to set the
  encoding globally.
* Added maven-source-plugin and maven-javadoc-plugin.
* Removed redundancy between plugins and pluginManagement.
* Removed Github downloads plugin.