Initial support for headers to set template engine.
Tests for setting the template engine using a header.
Constant in capitals.
Use of paths to engines in the configuration.
PEP8
Added a unit test, and bug corrected.
Psycho-rebased branch 77-template-engine-header on top of master
Refs #77 - Using 'engine' in query string instead of 'x-diecutter-template-engine' header to guess the engine to use. Deprecated 'template_engine' and 'filename_template_engine' headers in favor of 'engine' and 'filename_engine'.
Refs #77 - Restored doctests in Sphinx documentation. Using '.directory-tree' instead of '.diecutter-tree', as expected by piecutter.
Refs #77 - API index lists supported engines. Added documentation about supported engines.
Refs #77 - Updated CHANGELOG.
Client can ask diecutter server to use a specific template engine using 'engine' argument in query string. Closes #77. Closes #83.