Added VarDumpOptimizer / VarExportOptimizer . [ci skip]

Signed-off-by: Rack Lin <racklin@gmail.com>
Merge pull request #77 from racklin/enhancement/var_export

Added VarDumpOptimizer / VarExportOptimizer . [ci skip]
Update README.md
Defaults for string variables, better installation message [ci skip]
Merge branch 'master' of https://github.com/phalcon/zephir
Merge branch 'origem/master'