Complete re-write

Added
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1. html support
2. support pyexcel 0.2.0's generator output
3. pypy and pypy3 in test targets
4. support file stream and dot notation, e.g. pyexcel.Sheet.rst will return rst text representation of it.

Updated
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1. #8, write_header as an option(False) to disable header writing
2. the json output of multiple sheet book will be sorted by its sheet names.
3. No longer, pyexcel-text is pyexcel-io plugin but pyexcel.sources plugin.