Improve callable check when loading a route's action

When loading a route's action lazily, test for callability more
sensibly. Rely on is_callable() for this and stop sniffing the string
representations for '::'.

Allow both null and false to identify the default action.

fixes #1
Make HotMelt\Action::defaultAction() static

HotMelt\Action::defaultAction() should have been implemented as static
all along.

fixes #7
Merge branch 'bugfix/action-callables' into develop

Make HotMelt\Action::defaultAction() static (fixes #7).

Improve callable check when loading a route's action (fixes #1).

Allow both null and false to identify the default action.