Merge pull request #22 from toddrjen/props

Improve the handling of properties in Neo Core
improved fixed for ticket #38 --_check_time_in_range provoque rounding problem when using times with float32
improved spiketrain tests
implement fix and tests for issue 66 -- SpikeTrain defaults to float64 dtype rather than dtype of input array
fix dtype problem with axonio if listTag in header is empty
fix handling of NaN values in rawbinarysignalio
fix handling of NaN values in spike2io
fix inconsistent handling of channel_index as attribute/annotation in analogsignal
general spiketrain cleanups and performance improvements, resulting in an initialization speedup of at least two orders of magnitude