Added a new setting: MAX_UPLOAD_TOTAL_SIZE

Form now keeps track of the total size of the uploads to keep it under
a set limit (default to 10M) e.g. to keep the total size of the message
within some email server limits.

Thus might be backwards incompatible unless the total size is redefined.
Merge pull request #49 from 7mp/master

Added a new feature: maximum total upload file size (default: 10M).