Verifier computes the buffersize, useful when streaming multiple flatbuffers

Fixed bug with negative hex constants (2).

Close #3898

Don't crash if str is null

Is useful especially when we want to create a string from another message string that might be null.

Rename CONTRIBUTING to CONTRIBUTING.md

Fixed operator++. Added CreateXXX for vector types.

Added --escape-proto-identifiers command-line option.

Fix for issue #3922

Also, clean up redundant ';' at end of java classes.

same fix for general code generator

Fix for make

This avoid to put the pom.xml file into the source directory. Normally the pom file is in a parent (/parent) folder and it is not mixed with the java source code.
An other thing is: this will make import of the project more easy from a IDE.

The side effect is that the target folder where maven build artifacts will move from the <flatbuffers>/java/target to <flatbuffers>/target therefore the gitignore file has been updated in consequences.

Revert "Create a maven like project structure for java development. Make it OSGi compliant. Generate the flatbuffers code for testing (example)."

This reverts commit 9875b0e0.