LICENSE files

As per #41 request I'm adding a LICENSE.txt file at repo's root with Apache2 license (which is the preferred license for Android according to http://source.android.com/source/licenses.html) (see commit b1b3c4b).

I'm also renaming NOTICE file to a more verbose filename LICENSE.txt (commit 66b3767) and removing the empty MODULE_LICENSE_APACHE2 file (commit 6ec314c)