Implement contact manager methods for pending contacts.

.../org/briarproject/bramble/api/contact/ContactManager.java

...arproject/bramble/api/contact/HandshakeLinkConstants.java

...i/src/main/java/org/briarproject/bramble/util/Base32.java

.../main/java/org/briarproject/bramble/util/StringUtils.java

.../org/briarproject/bramble/contact/ContactManagerImpl.java

.../java/org/briarproject/bramble/contact/ContactModule.java

...g/briarproject/bramble/contact/PendingContactFactory.java

...iarproject/bramble/contact/PendingContactFactoryImpl.java

.../briarproject/bramble/contact/ContactManagerImplTest.java