Remove LICENSE.MPL2

Code depending on Eigen moved to jubatus_core, so remove LICENSE.MPL2 (jubatus_core has the file).
Modify description about Eigen in README.rst (Fix #826).

Eigen moved to jubatus_core, so updated about it.
Merge pull request #831 from gwtnb/documents-0.6.0

Update documents for 0.6.0 (Fix #826).