Added Net Templates to easy definition of Nets

This PR is inspired by #1290 and the discussion there. This PR also reuse parts of the code done by @cypof.

Instead of modifying `Net.Init()` this PR provides a new tool `./build/tools/expand_net` that is able to expand templates and do the need string substitutions. 
Usage: 
```
expand_net net_proto_file_in net_proto_file_out
```
For some template examples look at `examples/templates/'
```
./build/tools/expand_net examples/templates/example_net.template examples/templates/example_net.prototxt
```