#7 XDG directory support

[Keith Bowes]

Just a little patch I whipped up. It uses ~/.abook if it exists but if it doesn't, it tries to use $XDG_DATA_HOME/abook and ~/.local/share/abook if $XDG_DATA_HOME isn't defined.

Resolves https://sourceforge.net/p/abook/feature-requests/13/.

[Raphaël]

merged in 0.6.2