add support for multiple service providers

[GoogleCodeExporter]

The current implementation of Persistence overwrites the spi file if there are 
multiple implementations of the same provider.
The SPI specification allows multiple implementations to be listed.

See my modifications at 
https://github.com/zcourts/spi/commit/aef422509fc5c89b09d8a38bf0ab7d9b900b9b35#d
iff-2d22835cc53a7358ee3d55e07f385571R136 
for a fix

Original issue reported on code.google.com by court...@crlog.info on 7 Apr 2014 at 9:00