Add support for multiple classloaders for CoerceUtil.

[DennisMcWherter]

Addresses issues with: #689

In summary, BeanUtilsBean#getInstance returns a pseudo-singleton. Namely, it's based on the Thread.currentThread().getContextClassLoader().

This PR sets up CoerceUtil for each new classloader as necessary.

[coveralls]

Coverage increased (+0.05%) to 71.679% when pulling cfe02a9 on multiClassLoader into c421a63 on master.

[wcekan]

@DennisMcWherter Perhaps we should not hold on to a Classloader reference indefinitely. Could you instead stick that into a WeakReference<ClassLoader>?