Fix security issue, minor API reorganization

This release provides three significant changes:

• Fix a security issue (did not check for ".." in path, opening access to the whole file system).
• Reorganize the files and build to separate the test code from the echttp implementation.
• Refactor echttp_static.c as an optional extension to echttp.c (remove dependency in echttp.c).

The last changes causes a small incompatibility in the echttp API that may requires an application code change. This only impacts applications using echttp_route static(), which becomes echttp_static_route() and requires to include echttp_static.h.