DataType: Add 'Nothing' type
DataType: Add support for << operator
DataTypeHDF5: New function to get a DataType from H5

Will use class, size and sign to determine the corresponding
DataType.
DataTypeHDF5: Support 'bool' and implement it as bitfield
PropertyHDF5: remove obsolete Value IO methods
ValueHDF5: remove obsolete code
Value: new implementation as union-like class
[test] Value : add basic tests
Merge branch 'master' into value
[test] Reenable DataSet tests

We will need them for the Value functions in a bit
Value: add ctors for [const] char *
DataTypeHDF5: data_type_to_size add support for strings

Not sure if this is actually a good idea, but ok for now
hdf5::DataSet: support for reading and writing of Values

Actually vectors of values that is
[test] DataSet: Add basic tests for writing and reading values
Merge pull request #38 from gicmo/value

Implement Value as union-like class