Bulk v8 cleanup:
- conventions and consistency
- added some comments
- removed some dead code
- always use v8:: prefix and don't use namespace v8
- removed redundant arg handles in v8/mongo conversion functions
- removed redundant erase logic in loadStored()
- s/exec error:/JavaScript error:/
- in v8-specific files, changed jsassert to use uassert instead of verify
- replaced verify() w/ uassert and massert

Includes fix for SERVER-7751.