Add a new fetch/4 signature to handle fallbacks

[whitfin]

This fixes #126.

This will add a new fetch/4 function which has an explicit fallback argument, rather than dealing with options in the get/3 signature. This will temporarily remove default fallbacks on a cache state until they can be re-introduced after #132 is concluded.

[coveralls]

Coverage remained the same at 100.0% when pulling aa50943 on issue-126 into 06f68a5 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling caa5608 on issue-126 into 06f68a5 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 69572df on issue-126 into 06f68a5 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 69572df on issue-126 into 06f68a5 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 72ae7c1 on issue-126 into 06f68a5 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 1b95863 on issue-126 into 06f68a5 on master.