Adding tests for require cache behavior

This covers the new methods that we want to implement and tests the default behavior of Proxyquire

thlorenz/proxyquire#25
thlorenz/proxyquire#26
Added preserveCache and noPreserveCache methods

Defaults to preserveCache

thlorenz/proxyquire#25
thlorenz/proxyquire#26
Updated readme for preserveCache() and noPreserveCache()

thlorenz/proxyquire#25
thlorenz/proxyquire#26