Various CMake improvements

[dean0x7d]

Note: The changes are best viewed individually because the first commit has a bit of a messy diff.

1. add_subdirectory vs. find_package

The first commit makes sure that these two methods for including pybind11 are 100% interchangeable. This involves adding a BUILD_INTERFACE and a namespaced alias for the interface library.

Tests are added to ensure this. To avoid quadrupling some files, the directory structure was changed a bit. This yields a messy diff, but the changes are straightforward:

Before:

tests
|-- test_installed_module
|   |-- CMakeLists.txt
|   |-- main.cpp
|   \-- test.py
\-- test_installed_target
    |-- CMakeLists.txt
    |-- main.cpp
    \-- test.py

After:

tests
\-- test_cmake_build
    |-- installed_function/CMakeLists.txt
    |-- installed_target/CMakeLists.txt
    |-- subdirectory_function/CMakeLists.txt
    |-- subdirectory_target/CMakeLists.txt
    |-- main.cpp
    \-- test.py

Renamed test_install target to test_cmake_build because it's now also testing the non-installed subdirectory build.

2. pybind11_add_module and docs

See the documentation for this one. Everything about pybind11_add_module is pulled into one subsection and some duplicate function explanations are removed from other parts of the text. Same thing for configuration variables: consolidated to one subsection.

3. Rename pybind11::pybind11 -> pybind11::module

Embedding the interpreter could use a future target called pybind11::embedded, so it might be good to make room for it already. Hence, this proposed rename for the extension module target.

cc @loriab

4. make check

Runs all available tests with a common target name to help out packaging as suggested in #442 (comment). Could be expanded with embedded tests in the future. Individual targets remain for selective testing.

[wjakob]

Beautiful, thank you -- the build system looks much more consistent now, and the documentation you added reads very well. I'll rebase the individual commits so that the history is preserved.