Add CMake configuration for Firebird

[mloskot]

@vnaydionov has done some testing with Firebird for the recent Git master, applied fixes in code and Makefile.basic (see pull #56)) and confirmed it basically works.
So, it would be nice to have Firebird back and released in SOCI 3.2.0.

Before, Firebird needs to be configured for CMake:

1. Is there FindFirebird.cmake module provided by CMake 2.8.10+? If not, we need to find/create custom one in src/cmake/modules
2. Add src/cmake/dependencies/Firebird.cmake module and call find_package for Firebird.
3. Add Firebird to list of searched dependencies in src/cmake/SociDependencies.cmake. (BTW, a separate task, this should be reimplemented to automatically look for files in src/cmake/dependencies/)
4. Create simple src/backends/firebird/CMakeLists.txt and call soci_backend macro.
5. Create simple src/backends/firebird/test/CMakeLists.txt and call soci_backend_test macro.

[mloskot]

@vnaydionov proposed the update in the pull #60

[mloskot]

Closing as the CMake confing has been submitted by @vnaydionov in #60