Change fobos file syntax and behaviour

Change fobos file syntax: sections 'builder' and 'files' collapse
in only one section named 'default'. To have previous behaviour
fobos file must have only one section named 'default'. Add additional
behaviour to fobos file: introduce different 'modes' into singular
fobos file through section 'modes'. The new syntax is:

    [modes]
    modes = mymode1 mymode2 ...

    [mymode1]
    compiler = ...
    cflags = ...
    ...

    [mymode2]
    compiler = ...
    cflags = ...
    ...
To select the actual mode a new switch is added to CLI for both 'build'
and 'clean' modes:

- FoBiS.py build -mode mymode2 # build project using mymode2 options
- FoBiS.py clean -mode mymode2 # clean project using mymode2 options

If no mode is selected the first mode defined into 'modes' option of
'modes' section is used (in the example mymode1 is the default). If
fobos provides only one mode (previous behaviour) the new syntax is:

    [default]
    compiler = ...
    cflags = ...
    ...