manpage: Various fixes and updates.
Added assertions to confirm that the session was terminated cleanly
Set the title of the main toolbar
Make labels an even height

This prevents some of the blurring effects to the label outline
that can be seen when the outline falls over a half-pixel.
pv::DeviceManager now manages opening/closing devices

It also notifies pv::SigSession objects when their devices are
going to be cleared.
Stop capture before connecting to a device
Adapt to new <libsigrokdecode/libsigrokdecode.h> header.
PulseView depends on libsigrokdecode >= 0.2.0 (optional).

Currently libsigrokdecode support is not compiled in by default, though.
README: Update in preparation of the 0.1.0 release.
Add initial NEWS file.
manpage: Document --loglevel and filename argument.
Install the manpage upon 'make install'.

This uses the cmake 'GNUInstallDirs' module, which has variables for
various installation directories, including CMAKE_INSTALL_MANDIR.

This closes bug #76.