Removed unmaintained qmake project files
Merged SignatureBuilder into Signature

There was very little difference between these classes and it was a bit
cumbersome to use. Better to have just one Signature class.
Don't include a private header
Added missing includes
Updated CMake build files

Removed CMake related installations: this project doesn't have anything
related to CMake to install.

Removed (almost) empty CMakeLists.txt in src directory: it wasn't used
for anything.
Updated version to 0.21.0

The version tries to follow that of libgit2 somewhat. Let's see if it
works out.
Specify soversion separately from the version number

Start from 0.
Moved private class to "private" subdirectory
Don't install private headers

Separated private headers and sources from public ones. Only install the
public headers.
Update libgit2 submodule

This is a non-released (i.e. non-tagged) version of libgit2 but it
contains a few important fixes on top of the most recent libgit2 0.21.x
release.