CXX-270 Clarify Scons Targets

New Scons targets:

scons mongoclient (build libmongoclient.a)
scons install (install mongoclient, static or shared depends on existence of --sharedclient flag)

scons unit (run unit tests)
scons build-unit (build unit tests)

scons integration (run integration tests)
scons build-integration (build integration tests)

scons examples (run examples)
scons build-examples (build examples)

scons all = mongoclient + build-unit + build-integration + build-examples
scons test = unit + integration (should examples be in here as well?)

default target = mongoclient

also renamed check-install-mongoclient to check-install (this seems to only be used in the travis build)