Update lastOffset when reading
Allow arbitrary whitespace, not just newline, after xref

Fixes #27.
Refactor random data generation

Add new RandomDataProvider object and implement existing random number
generation in terms of that.  This enables end users to supply their
own random data providers.
Allow OS-provided secure random to be disabled
Retarget 5.1.0 changes to 5.2.0
Comments about incremental update support

Also remove some trivial, non-functional code.
TODO note on pluggable filters
Increase random data provider support

Add a method to get the current random data provider, and document and
test the method for resetting it.
Remove needless #ifdef _WIN32 from getWhoami
Prepare 5.1.0 release