Removes dirty checking from *Manager classes

Removes use of _dohash to check for dirtiness of attributes in the ParameterManager and MasterManager classes.  The functionality was intended to reduce redundant writing of attributes that hadn't changed since the last flush, however this has proven to be more costly then simply writing all attributes during each call to flush().

Updates tests accordingly.
Fixes test_coverage_basics setUp/tearDown

Addresses race condition with temporary directory creation/deletion.  Was causing h5py IOErrors because the temporary directory was being deleted before the test fully completed.
Sets chunks=None for hdf writes

chunks=None turns chunking off and improves read-times significantly for timeseries datasets.  This may become problematic with multidimensional datasets.
Checks for empty coverage in get_data_bounds

Adds a check at the beginning of get_data_bounds to raise a ValueError if the coverage is empty
Increases default brick size from 10k to 100k

Analysis of the effect of varying brick size indicate minimal (to no) performance gains with brick sizes > 100k
See: http://cl.ly/MXsy
Adds brick_size option to emptysamplecov