Make default thrift compiler location based off of THRIFT_HOME env variable

The maven build requires thrift to live in a particular location or be specified by the thrift.executable property at the command line. For example 'mvn -Dthrift.executable=/usr/local/bin/thrift'. This patch will attempt to use a THRIFT_HOME env variable as a default location for thrift. In the previous example THRIFT_HOME would be /usr/local.