Clarify naming and role of clean flag

Prior to this change, the "clean flag" was named `user.data.clean.dir`,
but in fact it cleaned the *app* data dir, not the *user* data dir.

This is the difference betwen deleting

    ~/Library/Application Support (the "user data dir")

vs.

    ~/Library/Application Support/Bitsquare (the "app data dir")

The name of this flag has been updated to `--app.data.dir.clean` to
reflect the fact that it cleans the latter vs. the former.

The processing of this flag has also been updated. It's default value
(false) is now assigned during the creation of the default properties
source in BitsquareEnvironment, and instead of reading and handling the
flag directly in the BitsquareEnvironment constructor, we now handle
cleaning in BitsquareApp#initAppDir, where logic for handling the
creation of the directory already exists.

See #291