Add parse_lambda and parse_pi

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add parse_theorem and parse_definition

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Update .travis.osx.yml and add scripts to run doxygen and testcov on travis-ci(OSX)
Update .travis.osx.yml and add scripts to run doxygen and testcov on travis-ci(OSX)