Only check time() once during _update()

Prevents inconsistencies
Encapsulate the price of a security

Decouple it from any specific monitor/exchange
Make test script output more comprehensive
Refactored tests to use new Price class
Prevent nose from running tests twice
Extended attributes tests to include new ones from Price class
Merge branch 'develop' into mtgoxmonitor
Set default updatePeriod for MtGox monitor
Refactored & extended MtGox monitor to use new Price class