First opam release candidate

Changes summary:

* More tests.

* The way values are stored changed: instead of indexing values with full path (tab1.tab2.(...).key.value), a tomlTable is a table of tomlEntries (indexed by their key): a direct value (TValue) or a subtable (TTable). It makes the structure a little bit more complex but it is really simpler to handle, especially when you want to retreive a list of subtables you do not know the name to iterate on it.

* Added some getters to help retreiving values from a table.

* Added method to read from a channel or a lexbuf.