Avoid throwing exceptions during lookup

[ghost]

Changes:

• SymSpellBuilder.java -- Make a bit more null-hostile, allow for multiple lexicon sources while building (e.g., standard English unigrams and user-configurable unigrams), prevent creating an instance of SymSpellImpl without a lexicon (to avoid having to check the lexicon on each lookup call).
• SymSpellImpl.java -- No longer throws exception on lookup; the lexicon must already exist by this point.
• SymSpellTest.java -- Test that building without a lexicon still goes boom.