make Mojo thread-safe.

This change lets user use -T option of mvn command.
To understand what thread-safe means in mojo's case, read
following document.
https://cwiki.apache.org/confluence/display/MAVEN/Parallel+builds+in+Maven+3#ParallelbuildsinMaven3-Mojothreadsafetyassertionchecklist
Merge pull request #21 from eller86/thread-safe

make StartMojo thread-safe.