Allow for empty class prefixes in config array.
Defaulting PEAR manager prefix to Net_Gearman_Job_

All the examples use Net_Gearman_Job_ as the prefix, so we should
default that.
Merge pull request #102 from jpirkey/fix-default-class-prefix

Fix default class prefix
Adding that 5.3 is the min version for composer.
Specifying the min php version composer supports