Implement SugarRecord.sum()

[SiebelsTim]

analog to count()

[mention-bot]

By analyzing the blame information on this pull request, we identified @whoshuu, @sibeliusseraphini and @RoyMontoya to be potential reviewers

[coveralls]

Coverage decreased (-1.04%) to 71.415% when pulling 92b675e on SiebelsTim:impl-sum into ce7d802 on satyan:master.

[JonatanSalas]

@SiebelsTim Why don't you add some test in order to not decrease the coverage?

[SiebelsTim]

@JonatanSalas At first I didn't find tests for count-like functions. After looking thoroughly, I added some tests :)

[SiebelsTim]

Btw: Should I call toSQLNameDefault()around the field names, or should I let the user change it to underscores?

[coveralls]

Coverage increased (+0.4%) to 72.849% when pulling f43255f on SiebelsTim:impl-sum into ce7d802 on satyan:master.

[coveralls]

Coverage increased (+0.4%) to 72.849% when pulling 59a2a91 on SiebelsTim:impl-sum into ce7d802 on satyan:master.

[sibelius]

LGTM

[sibelius]

thanks

[SiebelsTim]

Thanks :)