Added base files for the BaseConverter activity

These are the required files to provide the base converter activity.
Navigation is already connected throught the MainWindow class and the
manifest. The class is, however, an empty placeholder to be filled in.

AndroidManifest.xml:
    Added the activity tag for this class

res/layout/base_converter:
    created empty placeholder

res/layout/main.xml:
    added the button and the callback to push the new activity

res/values/strings.xml:
    Added the constant to show in the status bar for the new activity

src/gidaibero/android/matsol/BaseConverterActivity.java:
    Added base class

src/gidaibero/android/matsol/MainWindow.java:
    Added the connecting code.