Create schema doc from bytes not files

[stephen-soltesz]

Requiring field descriptions to be in files constrained the use case from m-lab/etl repo.

This change modifies the bqx SchemaDoc functions to accept bytes rather than files.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 589

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 82.434%

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Totals
Change from base Build 587:	-0.05%
Covered Lines:	840
Relevant Lines:	1019

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💛 - Coveralls