Fix util.is_mm_project() function

Added check for /config/.settings file, used for showing menus, etc
Implement is_visible() and is_enabled() for MavensMate commands

Programmatically enable and disable menu commands - hide when not a MM
project, disable conditionally on file type and contents.
Open on Server

Implementation of open_sfdc_url command for plugin. Conditionally
show/hide.