Adding support for nested columns to ptdump.

Ptdump failed to correctly write nested colums, which return a column
collection instead of a column instance via the _f_col property. Add explicit
check for column type during column dump.
Generate Float96 and Float128 if NumPy defines them.
Get Arrays atom type from the native type instead of the disk type.
Build tables columns from native data types.
Add floats float96 and float128 to the conversion table.
Merge pull request #213 from asford/master

Add support for nested columns to ptdump.
Fix keyword names in example code
Updated release notes for gh-213
Add tests for extended precision datatypes.
Fix typo in test_types.py
Fixed comments in test_types.py
Fix test_types.py logic.
Merge branch 'develop' of github.com:PyTables/PyTables into andreabedini-extended_precision