[docs] fix missing glog install cd
[docs] install glog first given incompatibility
[docs] compile gflags with -fPIC for linking
Add NetState message with phase, level, stage; NetStateRule message with
filtering rules for Layers.
Incorporate NetState{,Rule} into Solver/Net.  Net::FilterParam
includes/excludes layers based on whether the NetState meets each
layer's NetStateRule(s).
Use unified train/test nets in examples.