Pluggable fetch strategies

[woodhull]

This PR adds the ability to have different pluggable strategies for what to do when the rate limit is exceeded.

For now there are new strategies that sleep or schedule jobs to run in the future. Added these in order to deal with issues described here #2

[coveralls]

Coverage decreased (-3.6%) to 96.364% when pulling e308e54 on controlshift:pluggable-fetch-strategies into bcb524e on enova:master.

[coveralls]

Coverage remained the same at 100.0% when pulling ea6912b on controlshift:pluggable-fetch-strategies into bcb524e on enova:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 210e629 on controlshift:pluggable-fetch-strategies into bcb524e on enova:master.

[bf4]

@packrat386 PR is stale. Close or review and rebase?

[rnubel]

This PR is older than both my children combined, but are you still interested in having it merged? I have no objection to it, conceptually.

[lavaturtle]

I'd be interested in seeing this merged. It would allow a codebase I work on to switch back to the published version of the gem instead of this fork.