src/dfinstancelinux.cpp: checksum: readAll on Qt4

Less efficient, but QCryptographicHash doesn't support passing in
QIODevices on Qt4. Fixes #121.
Merge pull request #124 from Hello71/qt4hash

src/dfinstancelinux.cpp: checksum: readAll on Qt4
src/dfinstanceosx.mm: checksum: readAll on Qt4

Less efficient, but QCryptographicHash doesn't support passing in
QIODevices on Qt4.
Merge pull request #125 from Hello71/qt4hash

src/dfinstanceosx.mm: checksum: readAll on Qt4
Remove OS-specific entries from .gitignore.

.gitignore should only store items that are generated by the code in the
repository. If your operating systems generates temporary files, put
those in your .gitignore_global.