Adding oai_available() and check_oai()

[mbojan]

This addresses #36 .

• oai_available checks if OAI-PMH is available at URL by requesting Identify. Returns TRUE/FALSE. It is exported.
• check_oai calls testthat::skip if there is no OAI-PMH at URL. It is not exported as it is useful only in the testthat tests.

All the tests relying on some OAI-PMH service could now start with a call to check_oai.

[codecov-io]

Current coverage is 63.84% (diff: 0.00%)

Merging #41 into master will decrease coverage by 0.96%

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[sckott]

LGTM