Truncate field values to the maximum length allowed in Salesforce.

Adds a setting to control this behavior. Default is to not truncate.
t128: Fixed a minor typo in salesforce_sync/includes/salesforce_sync.admin.inc.
t128: Refactored SalesforceSync::truncateLongStrings() to differentiate between the maximum lengths of two fields with the same name on different SF object types.
Merge pull request #634 from JacksonRiver/7.x-4.x_17_truncate_long_strings

Truncate field values to the maximum length allowed in Salesforce.