Upgrade mim4j 0.6 -> 0.7.2

The recent upgrade of apache cxf to version 2.4.8 has a (loosely connected) dependency on upgrading mime4j.

 +- apache cxf 2.4.8
 |  \- neethi 3.0.2
 |     \- apache axiom 1.2.13 (provided and test scope deps)
 |        \- mime4j 0.7.2

This upgrade does involve some API changes but the module is marked "private" so it may or may not be acceptable for the 7.1 branch. The testsuite passes with the update, but the upgrade might require a resteasy upgrade because resteasy uses the mime4j api directly.