Update gif-clj 

Hi Robin,

Not sure if you are interested in maintaining/improving this package but I had a use case that called for the ability to change the delay on a per-frame basi: I wanted the last frame to linger for awhile before looping. I made some changes as outlined below and wanted to send them back to you incase you are interested updating/merging.

Changes:
1. Reorganized Java code to be on the classpath  
2. added the ability to change the delay setting for each frame
3.  set up the options in `write-gif` to specify the last frame separate from the rest
4.  document the updated write-gif function

regards,
zachcp