Resolve namespace issues

[n-samar]

Changes in this pull request resolve the following issues:

• C++ libraries should be wrapped in a namespace to avoid name conflicts with code that uses them. The entire project is wrapped into the heaan namespace.
• Header files should not use using directives as this pollutes the namespace of source files that include those header files. The using directives are removed from all header files and moved to implementation (.cpp) files.
• Some trailing whitespace removed.
• A file should #include all headers it uses, even when some other already-included header file includes it. This is the reason for the extra #include <string> in some places.
• test.cpp is modified to work with the new namespace. This is done by adding the appropriate using directives.
• Since most of the repo is tab-indented, occurrences of four spaces are changed to a tab.

[SHINJUNGWOO]

I think these requests are useful. When I used this library, I had an issue with the namespace. Using namespace std and ntl occurs some conflict problems in using other libraries.

[SHINJUNGWOO]

Thank you!