Maven build

This update adds a reference **pom.xml** build script to serve multiple purposes associated with the release process.

1. The **pom.xml** encodes the dependency information for the ANTLR 4 Tool and Runtime components which the Python code generator extends. Initially the build references the current 4.3 release of ANTLR.
2. The Maven build allows for easy publication of the Python 2 code generation components to Maven Central, which is a critical component of step 2 of the release process described in antlr/antlr4#640.
3. With the ANTLR dependency versions fixed in **pom.xml**, there is no need to implement `Target.getVersion` with a string literal. Instead, the compile-time constant `RuntimeMetaData.VERSION` will always provide the expected result. When updating this target to support a newer version of ANTLR 4, simply update the reference dependency version in **pom.xml** and the `Target.getVersion` value will be updated to match.
  * Note that the version number in **Recognizer.py** is not automatically updated, so it should be manually updated each time the version in **pom.xml** is updated (a rare occasion that only occurs during step 2 of the release process).
4. A complete jar file which supports code generation for Python 2 can be made available immediately. In other words, the Python 2 target is first introduced for ANTLR 4.3, and first *bundled* with ANTLR in 4.4.

If you have any concerns, or simply don't want to deal with Maven Central, I would be happy to handle the publication of the code generation components at any point when you feel the library is ready for release.