strip leading/trailing space before tokenisation

prevents stray empty strings from entering the index
implement sorted set indexOf as a binary search

results in vastly improved performance since we know the set elements
are sorted
expose stop words list as a sorted set

allows easy addition of extra stop words if required
using sorted set also results in greater performance
add JSON.stringify support

serialises the index and all its stores
add load to index

loads a previously serialised index
implement serialisation of the pipeline

pipeline functions need to be registerd if they are to be serialised
the pipeline will issue warnings about using unregistered pipeline functions
add an integration test covering serialisation
pre-build the example index
serialise documentation
build version 0.3.0