[#5] Add printer.

Add a Toml.Printer module to make it possible to serialize a TOML structure to
string, as a valid TOML document.
Stylistic improvements.
[#5] Removes the last uses of the Printf module in print_* functions.

This incidentally improves the display of floating numbers, and makes the
print_* functions more homogenous.
[#5] Removes wrong call to Key.of_string.
Merge pull request #8 from Emm/v2_printer

V2 printer