#374 added Card structure and the cards() method on HDU

[chrsoo]

PR that does not (yet) follow the contributing guidelines to facilitate the discussion on #374.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.21053% with 15 lines in your changes missing coverage. Please review.

Project coverage is 88.9%. Comparing base (53611ef) to head (c3a83e4).

Files with missing lines	Patch %	Lines
fitsio/src/headers/card.rs	64.2%	10 Missing :warning:
fitsio/src/hdu.rs	92.5%	5 Missing :warning:

Additional details and impacted files

Files with missing lines	Coverage Δ
fitsio/src/headers/mod.rs	82.8% <ø> (ø)
fitsio/src/hdu.rs	97.2% <92.5%> (-0.9%)	:arrow_down:
fitsio/src/headers/card.rs	64.2% <64.2%> (ø)