Add -pthread and remove -lpthread.

-pthread introduces -lpthread and -D_REENTRANT.
-D_REENTRANT is required for safety.
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).
Merge pull request #830 from gwtnb/pthread

Add -pthread.
add jubaserv_common_logger to pkgconfig manifest (refs #807)
fix include gurad errors
bump version
Merge branch 'release/0.6.0'
Merge branch 'release/0.6.0' into develop
bump RPM packaging version
Merge branch 'master' into develop
bump DEB packaging version
Fix an incorrect variable in jubapkg.
Port to Maverics.
cleanup proxy code
remove unused get_private_session_pool function
make async_task_loop::startup private
Fix cpplint.
Move error handling code to avoid undefined behavior.