Use -fPIC when possible.

So that we can link the library statically to shared libraries.
Added more detailed CMake documentation.

- Also added better FindSphinx support so that we can detect the version and not  have a fatal error if it's too old.
- Added support for building latex documentation with -DBUILD_LATEX. Off by default.
- Added suppor for building man pages with -DBUILD_MAN. On by default (unless sphins is too old to support it).
Fixed some minor issues.
Merge pull request #112 from JoakimSoderberg/fpic_cmake_support

Additional CMake documentation + Fpic cmake support