Introducing upload file handlers
default implemented file handlers are sb_file_upload_handler:temporary_file_handler() and sb_file_upload_handler:memory_file_handler()
fixed internal functions export
* setting default max size for uploaded files in memory to 0 for backwards compatiblity
* renaming constant for max file in memory size
Updated Readme for new file upload handling
Merge pull request #30 from synlay/feature/multipart_in_memory

Introducing memory file upload handling