Add ability to set main class name

Main class name is one of things I always change. This adds ability to do so
during project creation, in similar manner how jberkel/android-app.g8
does with main_activity variable. Here, it is main_class variable,
because it is shared by all platforms. I've left Main.* for each platform
how it is.
Small reordering in AndroidManifest

Did small cleanup in AndroidManifest, like adding some empty lines for spacing
and moving min SDK before target SDK to mirror order from documentation:
http://developer.android.com/guide/topics/manifest/uses-sdk-element.html
Added separate min_api_level

minSdkVersion is one of variables that are nearly always set to value lower
than targetSdkVersion, but present because otherwise it defaults to 1. It also
usually does not change during project, so it makes sense to set it up during
project creation. This variable is also included by the new Android build
scripts:

http://tools.android.com/tech-docs/new-build-system/user-guide#TOC-Basic-Build-Customization
Changed version in manifest to 0.1

Set version to 0.1 to mirror one set in project/build.scala. It also
makes more sense to start from 0.1 than from 1.0, so I believe it is
better default.
Moved proguard configuration to file

It is easier and more standard way to configure proguard than hacking our
way trough build.scala file.
Added all classes from main package to proguard

This seems like good default, at least for testing purposes.
Later on, users can customize this easily inside configuration file.
Made application debuggable by default

Setting application to debuggable seems to be better default
for starting fresh project. Easier to get started with and customize
later.
Merge pull request #14 from aginiewicz/manifest-features

Set of small features to make starting new development easier