Add a function for determining whether HyPackage is installed or not
Return the installed size for installed packages in hy_package_get_size()

- Kinda related to RhBug:968157, when erasing packages in dnf you'll
  now get an idea of how much space will be freed.
tests: fix a memory leak revealed by libsolv commit 0804020.
Add .baseurl getter to Python and C APIs.
tests: add test-case for hy_package_installed()
py: add 'installed' property to hawkey.Package
Merge remote-tracking branch 'hawkey/master'