Eliminate all uses of the current working directory, and
set up multiple directories specified by absolute paths for various uses.

Add a WorkingDir class to create a user config directory and
migrate files to it for new installs.
The directory will be $HOME/.i2p on linux and %APPDIR%\I2P on Windows,
or as specified in the system property -Di2p.dir.config=/path/to/i2pdir
All files except for the base install and temp files will be
in the config directory by default.
Temp files will be in a i2p-xxxxx subdirectory of the system temp directory
specified by the system property java.io.tmpdir.

Convert all file opens in the code to be relative to a specific directory,
as specified in the context. Code and applications should never open
files relative to the current working directory (e.g. new File("foo")).
All files should be accessed in the appropriate context directory,
e.g. new File(_context.getAppDir(), "foo").

The router.config file location may be specified as a system property on the
java command line with -Drouter.configLocation=/path/to/router.config
All directories may be specified as properties in the router.config file.

The migration will copy all files from an existing installation,
except i2psnark/, with the system property -Di2p.dir.migrate=true.
Otherwise it will just set up a new directory with a minimal configuration.

The migration will also create a modified wrapper.config and (on linux only)
a modified i2prouter script, and place them in the config directory.

There are no changes to the installer or the default i2prouter, i2prouter.bat,
i2prouter, wrapper.config, runplain.sh, windows service installer/uninstaller,
etc. in this checkin.


    *  Directories. These are all set at instantiation and will not be changed by
    *  subsequent property changes.
    *  All properties, if set, should be absolute paths.
    *
    *  Name	Property 	Method		Files
    *  -----	-------- 	-----		-----
    *  Base	i2p.dir.base	getBaseDir()	lib/, webapps/, docs/, geoip/, licenses/, ...
    *  Temp	i2p.dir.temp	getTempDir()	Temporary files
    *  Config	i2p.dir.config	getConfigDir()	*.config, hosts.txt, addressbook/, ...
    *
    *  (the following all default to the same as Config)
    *
    *  Router	i2p.dir.router	getRouterDir()	netDb/, peerProfiles/, router.*, keyBackup/, ...
    *  Log	i2p.dir.log	getLogDir()	wrapper.log*, logs/
    *  PID	i2p.dir.pid	getPIDDir()	wrapper *.pid files, router.ping
    *  App	i2p.dir.app	getAppDir()	eepsite/, ...
    *
    *  Note that we can't control where the wrapper actually puts its files.

All these will be set appropriately in a Router Context.
In an I2P App Context, all except Temp will be the current working directory.

Lightly tested so far, needs much more testing.