Introduce nordlicht_error(), which returns a description of the last error

Update the example. This idea is stolen from libgit2. Fixes #24.
Move display of "Building keyframe index..." to the tool

We lose the indexing progress this way, but the library shouldn't print
stuff.
Add a --quiet switch to the tool, that suppresses progress output
CMake: Check if help2man is installed at configure phase

Closes #29.
On Windows, use _fullpath() instead of realpath()
Move most of the README to http://nordlicht.github.io/

Main reason: Avoid having large images in this Git repository. On the homepage, high-resolution barcodes are okay.
Merge branch 'master' of github.com:blinry/nordlicht
Remove PNGs from the repository
Use avg_frame_rate instead of r_frame_rate

See this thread on why it was removed in libav:
https://lists.libav.org/pipermail/libav-devel/2012-June/029761.html

Furthermore, use new AVFrame API.

Fixes #31.
Merge branch 'master' of https://github.com/nordlicht/nordlicht into debian
import current master for snapshot build