Fixed various bugs in the build process

In the process of working on Quantimodo, we've fixed various bugs in the build process:

0. README.md now links to [the instructions on the Github wiki](https://github.com/fluxtream/fluxtream-app/wiki), since those are much more complete (@Olathe fixed).
0. Builds no longer fail if `git` command-line client isn't installed (like on [Codenvy](https://codenvy.com/) or when someone downloads a `.zip` file of the project from Github): in that case, `badbadbadbad` used as latest commit ID in `lastcommit.properties` (@Olathe fixed).
0. Builds no longer fail on environments without real shells (like Codenvy...or Windows): `maven/install.sh` was converted to `maven/pom.xml` (@Olathe fixed). This has nice side benefits:
 * It works flawlessly on Windows.
 * No more forgetting to run `install.sh` when the standalone JARs change on Github or when getting everything up and running.
 * It's so quick your build times will barely be affected.
 * Maven can autodetect duplicate JARs (it already found that `core-1.0.jar` was an outdated duplicate of `gdata-core-1.0.jar`).
0. Builds no longer give a warning about a malformed POM (@renfeng fixed).
0. Builds no longer give a warning about moving the `activation` library to `javax.activation` (@Olathe fixed).