Added behaviour directive

The `behaviour` directive is part of the standard OTP distribution. However, the syntax highlighting was not elegant, as the contents where highlighted as atoms:

``` erlang
-behaviour(foo_bar). % foo_bar highlighted as an atom
```

With this fix, the `behaviour` directive is highlighted exactly like the `module` directive, since it is within a similar context.

Source: http://www.erlang.org/doc/design_principles/des_princ.html#id59741