Suggest davfs2 install if not available in `box-sync check`

[vitorbrandao]

~ $ bin/box-sync setup
* Setting up davfs...
Created configuration file '/.noiselabs/box/box-sync.cfg'
Created example configuration file '/.noiselabs/box/box-sync.cfg.sample'
* Created sync directory at '/Box'
Traceback (most recent call last):
File "/projects/box-linux-sync/bin/box-sync", line 55, in <module>
    retval = box_main()
File "/projects/box-linux-sync/src/noiselabs/box/main.py", line 108, in box_main
    setup.wizard()
File "/projects/box-linux-sync/src/noiselabs/box/setup.py", line 197, in wizard
    self.setup_davfs()
File "/projects/box-linux-sync/src/noiselabs/box/setup.py", line 95, in setup_davfs
    if not user in grp.getgrnam("davfs2").gr_mem:
KeyError: 'getgrnam(): name not found: davfs2'
~ $ bin/box-sync check
* Checking davfs installation...
! Davfs is not installed in your system. Please install it and re-run this application.
You may install it using: $ sudo emerge davfs2