Fix compilation warning in SparseField::releaseBlocks()

Fixes Issue #71.
Fix boost dependency.
Make it work on Mac OS X 10.9.

Should be backwards compatible but have not tested it on other
OS X versions or platforms.
Bumping version to 1.4.1 with OSX 10.9 updates.
Replaced 'int' with 'int64_t' in SparseFile to prevent overflow numerical issues.
Also increased warning levels with '-Wextra' and fixed subsequent new warnings.
Fixed regression in SparseFieldIO: Block data was allocated even with dynamic loading.