Added eval() and a Read-Evaluate-Print-Loop example.
Available in examples/repl.txt
Fixed: Return from if/else/for/while block was invalidated.
Improved performance.
Eliminated trivial productions from the abstract syntax tree before running, to significantly improve performance.
Built without profiling
Update .travis.yml

* Use GoogleTest for unittest
* Run tests for duck/memory.c
Add CMakeLists.txt

* Use GoogleTest for unit tests
* Compile test program
  * run_memory_tests
Add memory_test.cpp
Rename "namespace" to "ref_namespace"

* Avoid to conflict with the C++ keyword
Merge pull request #12 from gregtour/unittest

Use GoogleTest for testing
REPL
This duck program provides a read-evaluate-print-loop for testing
programs with Duck. Ideally, this would be stored internally and ran if
the interpreter is started without arguments.