Fix #241: Getting key value pairs from the context

[Pfeil]

This adds a few methods to read from the crate context.

• get value to a given key (there is no way to modify or assign it)
• get immutable set of keys
• get immutable map of key-value-pairs

The immutability of the return values is given in the Javadocs.

[coveralls]

Pull Request Test Coverage Report for Build #354

Details

• 17 of 17 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 87.033%

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Totals
Change from base Build #348:	0.08%
Covered Lines:	1886
Relevant Lines:	2167

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💛 - Coveralls