TAP formatter

I write a [TAP](http://en.wikipedia.org/wiki/Test_Anything_Protocol) formatter, but I start by a refactoring of public interface.

The Test Anything Protocol comes from Perl QA ecosystem and allows a separation between test producers and consumers.

The standard consumer is `prove`, and its usage with `specl` is :
`$ prove --exec="specl --formatter=specl.formatter.tap" specs/*_spec.yaml`
or
`$ prove -v --exec="specl --formatter=specl.formatter.tap" specs/*_spec.yaml`

Note: I've written the TAP producer [lua-TestMore](http://fperrad.github.com/lua-TestMore/).