Temporary path, updated requirements, renaming files.

[jacksonj04]

Just a collection of useful changes we've made to the library. Feel free to cherry pick individual changes if you don't want the whole lot.

• Added a setter for the temporary path (including sanity checking).
• Moved temporary path variable from private to protected.
• Removed exec() requirement.
• Added PHP ZIP requirement.
• Clarified filesystem access requirements.
• Added ability to specify a 'destination' name for files when being added (eg person_name.png to thumbnail.png)