Add new AppDataDir function.

This commit adds a new AppDataDir function that can be used to get an
operating system and application specific directory to be used for storing
application data.

For example:

dir := AppDataDir("myapp", false)

Would result in:

POSIX (Linux/BSD): ~/.myapp
Mac OS: $HOME/Library/Application Support/Myapp
Windows: %LOCALAPPDATA%\Myapp
Plan 9: $home/myapp

This is work toward conformal/btcd#30.